Unicorn Formatting Objects (UFO), Version 1.00.01
Support for XSL Formatting Objects and Properties
TeX Back-End
Created: 04-Sep-2000
Revised: 15-Sep-2000
Last revision: 29-Sep-2000
Copyright (C) 2000, Unicorn Enterprises SA
All rights reserved
1 Introduction
--------------
This document defines the subset of XSL formatting objects and
their properties as supported by UFO version 1.00.01 using the
TeX back-end.
The detailed description of the TeX back-end interfaces and
implementation is available as a separate document included
in the product distribution:
Unicorn Formatting Objects (UFO), Version 1.00.01
Specification for TeX Back-End
Starting with version 1.00.01, UFO supports also HTML back-end.
This back-end is not discussed here. The detailed description
of HTML back-end features is available as a separate
document included in the product distribution:
Unicorn Formatting Objects (UFO), Version 1.00.01
Support for Formatting Objects and Properties
HTML Back-End
2 General Notes
---------------
Notational Conventions
----------------------
Known unsupported yet important features are marked with the
ISSUE prefix in this document. In most cases, the decent effort
will be made to implement them in the near future.
ISSUE: Create a summary of all current ISSUEs at the end of
this document.
Front-end and Back-end
----------------------
The XSL processing part consists of the front-end (written in C++)
and the back-end (written in TeX macro language with the substantial
use of LaTeX packages). The front-end generates the sequence
of TeX macro calls from the source XML file; the back end drives
the TeX formatting engine interpreting macros generated
by the front-end.
Version of XSL Specification
----------------------------
The current release is based on the W3C specification
"Extensible Stylesheet Language (XSL) Version 1.0"
W3C Working Draft 27 March 2025
(http://www.w3c.org/TR/2000/WD-xsl-20000327)
No claims are made about the conformance.
Use of CSS2 Specification
-------------------------
XSL specification does not define many important features.
Instead, explicit (and in some cases implicit) references are
made to CSS2 specification. Implementation of such features
is based on the W3C specification
"Cascading Stylesheets, Level 2 (CSS2)"
W3C Recommendation 12 May 2024
(http://www.w3c.org/TR/1998/REC-CSS2-19980512)
Formatting Objects, Properties and Property Values
--------------------------------------------------
The following formatting objects are currently supported:
fo:root
fo:declarations
fo:page-sequence
fo:layout-master-set
fo:page-sequence-master
fo:single-page-master-reference
fo:repeatable-page-master-reference
fo:simple-page-master
fo:region-body
fo:region-before
fo:region-after
fo:flow
fo:static-content
fo:block
fo:character
fo:external-graphic
fo:inline
fo:leader
fo:page-number
fo:table-and-caption
fo:table
fo:table-column
fo:table-caption
fo:table-header
fo:table-footer
fo:table-body
fo:table-row
fo:table-cell
fo:list-block
fo:list-item
fo:list-item-body
fo:list-item-label
fo:wrapper
For all other formatting objects, UFO implements the appropriate
fallbacks according to provisions of XSL WD (Chapter 8,
"Conformance").
Detailed discussion of support for all formatting objects can be
found in the following sections of this document. For each object
type the list of supported properties is given as well.
For unsupported formatting objects, the fallback implementation
is described.
Units of Measure
----------------
All units of measure defined by XSL except 'px' (pixels) are
supported. In the internal calculations as well as in the output
TeX code, for all properties except 'font-size', units of measure
are converted to millimeters. For the 'font-size' property, units
of measure are converted to points.
Color Values
------------
Hexadecimal color values having the format
#[a-fA-F0-9]
are supported. Maximum of 6 hexadecimal digits allowed. These
values represent RGB colors in the way similar to one described in
HTML 4.0 specification.
The following named color values are supported:
aqua #00FFFF
black #000000
blue #0000FF
fuchsia #FF00FF
gray #808080
green #008000 [sic!]
lime #00FF00
maroon #800000
navy #000080
olive #808000
purple #800080
red #FF0000
silver #C0C0C0
teal #008080
white #FFFF00
An additional color value
orange #FF8000
is supported for interoperability with the PassiveTex package.
Percentages
-----------
Percentages in property values are supported when applicable.
Shorthand Properties
--------------------
The following shorthand properties are supported
border-width
border-color
border-style
border-top
border-right
border-bottom
border-left
border
font
margin
padding
white-space
The 'vertical-align' is implemented as a normal (not a shorthand)
property and is currently supported for 'fo:external-graphic' only.
Expressions
-----------
XSL expressions are not implemented. The only exception is the
minus sign that can precede numeric values.
Inheritance
-----------
Property inheritance is supported. A property that is implemented
in this release may be assigned to a formatting object of any type
allowed by XSL/CSS2, even if this formatting object makes no direct
use of this property. The property value may be inherited and used
by some child formatting object. In this document, properties of this
kind are mentioned explicitly only for the formatting object types
that make the direct use of them; however, for inheritance purposes
these properties are supported also for other object types when
allowed by XSL/CSS2.
Writing Modes
-------------
The 'writing-mode' property is not supported.
By default, the 'lr-tb' writing mode is assumed. The relative margin,
border and padding properties that contain 'before', 'after',
'start' and 'end' in their names are interpreted according to this
writing mode.
Margin Properties
-----------------
The following margin properties are supported:
margin-top
margin-bottom
margin-left
margin-right
margin-before
margin-after
margin-start
margin-end
space-before
space-before.optimum
space-after
space-after.optimum
In the sections that describe properties supported for each
formatting object type, these properties are referenced collectively
as "Margin Properties".
Properties 'space-before' and 'space-before.optimum' are interpreted
as corresponding to 'margin-top'; properties 'space-after' and
'space-after.optimum' are interpreted as corresponding to
'margin-bottom'.
Margin collapsing/space resolution is not supported.
ISSUE: Margin collapsing/space resolution is an important
feature. It should be implemented in the near future.
Border Properties
-----------------
The following border properties are supported:
border-top-style
border-bottom-style
border-left-style
border-right-style
border-before-style
border-after-style
border-start-style
border-end-style
border-top-color
border-bottom-color
border-left-color
border-right-color
border-before-color
border-after-color
border-start-color
border-end-color
border-top-width
border-bottom-width
border-left-width
border-right-width
border-before-width
border-after-width
border-start-width
border-end-width
All XSL border styles are accepted, but all styles other than
'none' and 'hidden' are interpreted as 'solid'.
In the sections that describe properties supported for each
formatting object type, these properties are referenced collectively
as "Border Properties".
Padding Properties
------------------
The following padding properties are supported:
padding-top
padding-bottom
padding-left
padding-right
padding-before
padding-after
padding-start
padding-end
In the sections that describe properties supported for each
formatting object type, these properties are referenced collectively
as "Padding Properties".
Font Properties
---------------
The following font properties are supported:
font-family
font-style
font-variant
font-weight
font-size
In the sections that describe properties supported for each
formatting object type, these properties are referenced collectively
as "Font Properties".
Currently only 35 standard PostScript fonts are supported.
(This is lie. Few additional PostScript fonts available in
NFSS packages provided with MiKTeX 1.20d are supported as well.)
ISSUE: It is important to support installable PostScript fonts.
This should be implemented in the near future.
The font selection scheme described in CSS2 is supported.
However, due to the limited number of PostScript fonts currently
supported not all combinations of font properties are allowed.
The following generic font families are supported
serif Times
sans-serif Helvetica
cursive ZapfChancery
monospace Courier
The 'fantasy' generic font family is not supported since there
is no true fantasy font among 35 standard PostScript fonts.
Text Align
----------
The 'text-align' property is supported in block, external graphic
and table cell formatting objects. The following property values
are supported: 'left', 'right', 'center' and 'justify'.
Properties Related to White Spaces
----------------------------------
The following properties related to white spaces are supported:
linefeed-treatment
space-treatment
white-space-collapse
wrap-option
The property 'wrap-option' is supported in a special way.
If this property has the value of 'no-wrap', it is interpreted
as if 'linefeed-treatment' would be set to 'preserve' (the
actual value of 'linefeed-treatment' is ignored in this case).
Breaking Formatting Objects at Page Boundaries
----------------------------------------------
If a formatting object is mapped to a TeX box or if a formatting
object is contained in a formatting object that is mapped to
a TeX box, such formatting object cannot be split at the page
boundary.
The following formatting objects are mapped to TeX boxes:
The 'fo:block', the 'fo:table-and-caption' or the 'fo:table-caption'
object that either:
- has 'height' property other than 'auto'
- has 'background-color' property other than 'transparent'
- has left and/or right border specified
The 'fo:table' object that either:
- is contained in another 'fo:table' object
- is contained a in formatting object that is mapped to a TeX box
The 'fo:cell' object.
The 'break-before' and 'break-after' properties are not interpreted
for formatting objects that are contained in formatting objects that
cannot be broken at page boundaries.
Advanced Typesetting Features
-----------------------------
Some advanced features important for typesetting applications are
not yet supported. This include (at least) multi-column output,
footnotes, subscript/superscript, keeps, orphans and widows.
ISSUE: These features should be implemented in the near future.
Language-dependent Features
---------------------------
Features, dependent on the language (hyphenation, etc.) are not
supported.
ISSUE: Due to the importance of these features, the Babel package
should be employed in the future release of the TeX
back-end. The corresponding XSL properties ('language', etc.)
should be implemented.
3. Declaration and Pagination and Layout Formatting Objects
-----------------------------------------------------------
Introduction
------------
Only A4 output media format is currently supported.
ISSUE: Other common media formats are important as well.
Media formats supported by LaTeX, PDFTEX and the
most popular DVI drivers should be implemented
in the near future.
Sequence page masters are implemented. Each page master may
indirectly reference several instances of 'fo:region-body',
several instances of 'fo:region-before' and several instances
of 'fo:region-after' formatting objects. However, currently
the following limitations are imposed:
- if several instances of 'fo:region-body' are referenced
by some page master, all of them must have the same
calculated values of 'margin-top', 'margin-bottom',
'margin-left' and 'margin-right' properties
- if several instances of 'fo:region-before' are referenced
by some page master, all of them must have the same
calculated values of 'extent' property
- if several instances of 'fo:region-after' are referenced
by some page master, all of them must have the same
calculated values of 'extent' property
Several page sequences may be contained in a source document.
Individual page sequences may use page masters that define different
dimensions of page regions (i.e., that define different calculated
values for 'margin-top', ..., 'extent' for page regions).
However, the current implementation of the TeX back-end may produce
wrong results (bad footer position) at the first page of the
page sequence that generates changes in the region dimensions.
(But several page sequences that reference the same page master
or page masters that describe the same region dimensions, will
be rendered correctly).
fo:root
-------
No individual properties supported.
fo:declarations
---------------
No individual properties supported.
fo:color-profile
----------------
This formatting object class is handled with the fallback.
fo:page-sequence
----------------
The following properties are supported:
master-name
initial-page-number
Numeric values as well as 'auto', 'auto-odd' and 'auto-even' values
are supported for 'initial-page-number' property.
fo:layout-master-set
--------------------
No individual properties supported.
fo:page-sequence-master
-----------------------
The following properties are supported:
master-name
fo:single-page-master-reference
-------------------------------
The following properties are supported:
master-name
fo:repeatable-page-master-reference
-----------------------------------
The following properties are supported:
master-name
maximum-repeats
fo:repeatable-page-master-alternatives
--------------------------------------
This formatting object class is handled with the fallback.
The page-master referenced in the first
'fo:conditional-page-master-reference' child is used.
The following properties are supported:
maximum-repeats
fo:conditional-page-master-reference
------------------------------------
This formatting object class is handled with the fallback.
The page-master referenced in the first
'fo:conditional-page-master-reference' child is used.
The following properties are supported:
master-name
fo:simple-page-master
---------------------
The following properties are supported:
master-name
page-height
page-width
Margin Properties
In the current release properties 'page-height' and 'page-width' are
allowed to have the 'auto' value only. The page format is always A4.
fo:region-body
--------------
The following properties are supported:
region-name
Margin Properties
fo:region-before
----------------
The following properties are supported:
region-name
extent
fo:region-after
---------------
The following properties are supported:
region-name
extent
fo:region-start
---------------
This formatting object class is handled with the fallback.
The content of the corresponding 'fo:static-content'
object is ignored. This fallback implementation is different
from that recommended by XSL WD.
The following properties are supported:
region-name
extent
fo:region-end
-------------
This formatting object class is handled with the fallback.
The content of the corresponding 'fo:static-content'
object is ignored. This fallback implementation is different
from that recommended by XSL WD.
The following properties are supported:
region-name
extent
fo:flow
-------
No individual properties supported. The 'flow-name' property,
if specified, will be ignored by the front-end.
fo:static-content
-----------------
The following properties are supported:
flow-name
fo:title
--------
This formatting object is ignored, together with its content.
4. Block-level Formatting objects
---------------------------------
Introduction
------------
Algorithms described in CSS2 are used to calculate vertical
and horizontal parameters of block objects (height, width,
margins, border-widths, paddings); the support for 'auto'
property values is provided when appropriate.
fo:block
--------
The following properties are supported:
height
width
min-height
min-width
max-height
max-width
background-color
Margin Properties
Border Properties
Padding Properties
Font Properties
color
linefeed-treatment
space-treatment
text-align
text-indent
white-space-collapse
wrap-option
break-before
break-after
ISSUE: The 'line-height ' property is not yet supported. Due to the
importance of this property, it should be implemented in
the near future.
fo:block-container
------------------
This formatting object class is handled with the fallback.
The processor displays indication that content cannot be correctly
rendered.
5. Inline-level Formatting Objects
----------------------------------
fo:bidi-override
----------------
This formatting object class is handled with the fallback.
The processor displays indication that content cannot be correctly
rendered.
fo:character
------------
The following properties are supported:
character
Font Properties
color
fo:initial-property-set
-----------------------
This formatting object class is handled with the fallback.
Any properties specified on this object are ignored.
fo:external-graphic
-------------------
The following properties are supported:
height
width
content-height
content-width
text-align
vertical-align
src
Percentage values in 'content-height' and 'content-width' properties
are supported. However, if one of this properties has the percentage
value, the other property must be assigned either percentage or 'auto'.
Percentage and length values of 'vertical-align' property are not
supported. All string values specified by XSL WD are recognized but
interpreted as follows:
baseline interpreted as bottom
middle middle
sub bottom
super top
text-top top
text-bottom bottom
top top
bottom bottom
The 'src' property value is interpreted as a local file system file
path specification rather than URI.
ISSUE: It must be URI (to do in the near future).
The set of supported graphical formats depends on the DVI driver used
to render a document. Please, consult the local driver documentation.
(And, if the particular DVI documentation appears to be ambiguous
or incomplete, look directly into "*.DEF" files distributed with
LaTeX2e graphics bundle).
It is known that (at least in the MiKTeX 1.20d distribution) DVIPS
supports (among others) EPS (Encapsulated PostScript) and BMP formats,
and PDFTEX supports (among others) PDF, JPG and PNG formats.
fo:instream-foreign-object
--------------------------
This formatting object class is handled with the fallback.
The processor displays indication that content cannot be correctly
rendered.
fo:inline
---------
The following properties are supported:
Font Properties
color
fo:inline-container
-------------------
This formatting object class is handled with the fallback.
The processor displays indication that content cannot be correctly
rendered.
fo:leader
---------
The following properties are supported:
leader-pattern
rule-style
rule-thickness
The value 'use-content' of the 'leader-pattern' property is not
supported and is interpreted as 'space'.
Values 'double', 'groove' and 'ridge' of the 'rule-style' property
are interpreted as 'solid'.
fo:page-number
--------------
The following properties are supported:
Font Properties
color
Only Arabic representation of page numbers is supported.
fo:page-number-citation
-----------------------
This formatting object class is handled with the fallback.
The processor displays indication that content cannot be correctly
rendered.
6. Table Formatting Objects
---------------------------
Introduction
------------
Support for tables with borders and padding is implemented.
Certain limitations are imposed on the table layout by the
current front-end implementation. Table columns must have
the fixed width explicitly specified. For each table column
the corresponding 'fo:table-column' object must be specified
with the explicit length value of the 'column-width' property.
Each 'fo:table-cell' formatting object must be contained
in the 'fo:table-row' object. Explicit height and width for
the entire table are not supported; for each 'fo:table' object
the 'width' property must be 'auto' and the 'height' property
is ignored by the front-end. Heights and widths specified
individually for table cells are not supported.
Collapsing borders are supported. Separated borders are NOT supported.
The border collapsing algorithm is based on CSS2 guidelines. However,
CSS2 provides no specification for several important aspects of
border collapsing. For instance, it is not specified how to collapse
adjacent borders of two cells that are different only in color.
There is also no detailed specification about rendering of adjacent
cell corners. The front-end tries to make consistent interpretation
in the situations not specified by CSS2, but the output may look
terrible in the certain (fortunately, quite exotic) cases.
There is no collapsing implemented for the adjacent borders between
header and body, body and body, body and footer.
ISSUE: Do we need implementation for the separated border model?
fo:table-and-caption
--------------------
The following properties are supported:
background-color
Margin Properties
Border Properties
Padding Properties
The 'caption-side' property is not supported. The table caption,
if specified, is always rendered before the table.
fo:table
--------
The following properties are supported:
width
background-color
Margin Properties
Font Properties
color
break-before
break-after
The only supported value of the 'width' property is 'auto'.
The 'height' property, if specified, is ignored by the front-end.
fo:table-column
---------------
The following properties are supported:
column-width
background-color
Border Properties
For each column of the table the corresponding 'fo:table-column'
object is required.
The 'column-width' property is mandatory and must specify the
explicit length.
fo:table-caption
----------------
The following properties are supported:
height
min-height
min-width
background-color
Border Properties
Padding Properties
In the current implementation, the table caption is always rendered
before the table.
fo:table-header
---------------
The following properties are supported:
background-color
Border Properties
fo:table-footer
---------------
The following properties are supported:
background-color
Border Properties
fo:table-body
-------------
The following properties are supported:
background-color
Border Properties
fo:table-row
------------
The following properties are supported:
height
background-color
Border Properties
If the 'height' property contains the explicit length,
it is used to specify the exact row height. If this property
has the value of 'auto', the row height will be calculated
by the back-end automatically.
fo:table-cell
-------------
The following properties are supported:
background-color
Border Properties
Padding Properties
Font Properties
color
linefeed-treatment
space-treatment
text-align
text-indent
white-space-collapse
wrap-option
Each 'fo:table-cell' object must be a child the 'fo:table-row' object.
The 'height' and 'width' properties, if specified, are ignored
by the front-end.
Cells spanning several columns and/or rows are not yet implemented.
ISSUE: Cells spanning several COLUMNS may be implemented in the
near future. Implementing cells spanning several ROWS
may be tricky and probably should be delayed for a distant
future. By the way, existing specifications provide no
reasonable definition for spanning cells. For instance,
what about the border collapsing for the adjacent spanning
cells in general case?
7. List Formatting Objects
--------------------------
The complete set of list formatting objects is supported.
In the current release, the horizontal layout of a list is fully
controlled by the 'margin-left', 'margin-right',
'provisional-label-separation' and
'provisional-distance-between-starts' properties as follows:
The distance between the left edge of the containing block and
the left edge of the label if equal to 'margin-left'.
The label width if equal to 'provisional-distance-between-starts'
minus 'provisional-label-separation'.
The distance between the right edge of the label and the left
edge of the body is equal to 'provisional-label-separation'.
The block width is equal to the width of the containing block
minus 'margin-left' minus 'provisional-distance-between-starts'
minus 'margin-right'.
The distance between the right edge of the body and the right edge
of the containing block is equal to 'margin-right'.
fo:list-block
-------------
The following properties are supported:
Margin Properties
provisional-label-separation
provisional-distance-between-starts
break-before
break-after
ISSUE: The support for 'background-color', Border Properties and
Padding Properties should be implemented at the same extent
as it was done for 'fo:block'.
fo:list-item
------------
The following properties are supported:
Margin Properties (vertical margins only)
break-before
break-after
ISSUE: The support for 'background-color', Border Properties and
Padding Properties should be implemented at the same extent
as it was done for 'fo:block'.
fo:list-item-block
------------------
No individual properties supported.
Note that the 'start-indent' property, as well as 'body-start()'
function suggested by XSL WD are not supported. The XSL WD
specification on this subject appears to be somewhat controversial,
since 'start-indent' is not listed among the properties allowed for
'fo:list-item-block'.
fo:list-item-label
------------------
No individual properties supported.
Item labels are treated as a boxes (and therefore cannot be split
between pages).
Note that the 'end-indent' property, as well as 'label-end()'
function suggested by XSL WD are not supported. The XSL WD
specification on this subject appears to be somewhat controversial,
since 'end-indent' is not listed among the properties allowed for
'fo:list-item-label'.
8. Link an Multi Formatting Objects
-----------------------------------
fo:simple-link
--------------
This formatting object class is handled with the fallback.
Content is promoted to the parent formatting object.
fo:multi-switch
---------------
This formatting object class is handled with the fallback.
Contents of the first eligible multi-case formatting object
is utilized.
fo:multi-case
-------------
This formatting object is used as a wrapper for the fallback
for multi-switch.
The following properties are supported:
starting-state
fo:multi-toggle
---------------
This formatting object class is handled with the fallback.
Content is promoted to the parent formatting object.
fo:multi-properties
-------------------
This formatting object class is handled with the fallback.
Content is promoted to the parent formatting object.
fo:multi-property-set
---------------------
This formatting object class is handled with the fallback.
It is ignored by the processor.
9. Out-of-Line Formatting Objects
---------------------------------
fo:float
--------
This formatting object class is handled with the fallback.
It is placed inline.
fo:footnote
-----------
This formatting object class is handled with the fallback.
It is placed inline.
fo:footnote-body
----------------
This formatting object class is handled with the fallback.
It is placed inline.
10. Other Formatting Objects
----------------------------
fo:wrapper
----------
This formatting object may be used to specify inherited properties
for a group of formatting objects.
fo:marker
---------
This formatting object class is handled with the fallback.
It is ignored by the processor.
fo:retrieve-marker
------------------
This formatting object class is handled with the fallback.
The processor displays indication that content cannot be correctly
rendered.
Appendix A. New in this Release
-------------------------------
Since 12-Sep-2000
-----------------
Added support for the variable (auto) height of 'fo:table-row'.
Added support for the following properties:
linefeed-treatment
space-treatment
text-indent
white-space-collapse
Added support for the following shorthand properties:
border-width
border-color
border-style
border-top
border-right
border-bottom
border-left
border
font
margin
padding
white-space
Added support for percentage property values.
Since 15-Sep-2000
-----------------
List Formatting Objects
fo:list-block
fo:list-item
fo:list-item-label
fo:list-item-body
and related properties
provisional-distance-between-starts
provisional-label-separation
are implemented.
Implementation problems of 'fo:external-graphic' are fixed.
The 'text-align' and 'vertical-align' properties are implemented
for 'fo:external-graphic'.
Since 29-Sep-2000
-----------------
The following formatting objects are implemented:
fo:declarations
fo:character
fo:leader
fo:table-and-caption
fo:table-caption
fo:wrapper
Several additional properties are related to these objects are
implemented as well.
Fallbacks implemented for all unsupported formatting objects.