rfc1942.txt

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Raggett                       Experimental                     [Page 22]

RFC 1942                      HTML Tables                       May 1996


   ROWSPAN, e.g. <TD ROWSPAN=2>
       A positive integer value that defines how may rows this cell
       spans. The default ROWSPAN is 1. ROWSPAN=0 has a special
       significance and implies that the cell spans all rows from the
       current row up to the last row of the table.

   COLSPAN, e.g. <TD COLSPAN=2>
       A positive integer value that defines how may columns this cell
       spans. The default COLSPAN is 1. COLSPAN=0 has a special
       significance and implies that the cell spans all columns from
       the current column up to the last column of the table.

   ALIGN, CHAR, CHAROFF and VALIGN
       Specify values for horizontal and vertical alignment within
       table cells. See inheritance order of alignment properties.

   Note: It is recommended that implementors provide support for the
   Netscape 1.1 WIDTH attribute for TH and TD, although this isn't part
   of the current specification. Document authors are advised to use
   the width attribute for the COL element instead.

Recommended Layout Algorithms

   If the COLS attribute on the TABLE element specifies the number of
   columns, then the table may be rendered using a fixed layout,
   otherwise the autolayout algorithm described below should be used.

Fixed Layout Algorithm

   For this algorithm, it is assumed that the number of columns is
   known. The column widths by default should be set to the same size.
   Authors may override this by specifying relative or absolute column
   widths, using the COLGROUP or COL elements. The default table width
   is the space between the current left and right margins, but may be
   overridden by the WIDTH attribute on the TABLE element, or determined
   from absolute column widths. To deal with mixtures of absolute and
   relative column widths, the first step is to allocate space from the
   table width to columns with absolute widths. After this, the space
   remaining is divided up between the columns with relative widths.

   The table syntax alone is insufficient to guarantee the consistency
   of attribute values. For instance, the number of columns specified by
   the COLS attribute may be inconsistent with the number of columns
   implied by the COL elements. This in turn, may be inconsistent with
   the number of columns implied by the table cells. A further problem
   occurs when the columns are too narrow to avoid overflow of cell
   contents. The width of the table as specified by the TABLE element or
   COL elements may result in overflow of cell contents. It is



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RFC 1942                      HTML Tables                       May 1996


   recommended that user agents attempt to recover gracefully from these
   situations, e.g. by hyphenating words and resorting to splitting
   words if hyphenation points are unknown.

   In the event that an indivisible element causes cell overflow, the
   user agent may consider adjusting column widths and re-rendering the
   table. In the worst case clipping may be considered if column width
   adjustments and/or scrollable cell content are not feasible. In any
   case if cell content is split or clipped this should be indicated to
   the user in an appropriate manner.

Autolayout Algorithm

   If the COLS attribute is missing from the table start tag, then the
   user agent should use the following autolayout algorithm. It uses two
   passes through the table data and scales linearly with the size of
   the table.

   In the first pass, line wrapping is disabled, and the user agent
   keeps track of the minimum and maximum width of each cell. The
   maximum width is given by the widest line. As line wrap has been
   disabled, paragraphs are treated as long lines unless broken by <BR>
   elements. The minimum width is given by the widest word or image etc.
   taking into account leading indents and list bullets etc. In other
   words, if you were to format the cell's content in a window of its
   own, determine the minimum width you could make the window before the
   cell begins to overflow. Allowing user agents to split words will
   minimize the need for horizontal scrolling or in the worst case
   clipping of cell contents.

   This process also applies to any nested tables occuring in cell
   content. The minimum and maximum widths for cells in nested tables
   are used to determine the minimum and maximum widths for these tables
   and hence for the parent table cell itself. The algorithm is linear
   with aggregate cell content, and broadly speaking independent of the
   depth of nesting.

   To cope with character alignment of cell contents, the algorithm
   keeps three running min/max totals for each column: Left of align
   char, right of align char and un-aligned. The minimum width for a
   column is then: max(min_left + min_right, min_non-aligned).

   The minimum and maximum cell widths are then used to determine the
   corresponding minimum and maximum widths for the columns. These in
   turn, are used to find the minimum and maximum width for the table.
   Note that cells can contain nested tables, but this doesn't
   complicate the code significantly. The next step is to assign column
   widths according to the available space (i.e. the space between the



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RFC 1942                      HTML Tables                       May 1996


   current left and right margins).

   For cells which span multiple columns, a simple approach, as used by
   Arena, is to evenly apportion the min/max widths to each of the
   constituent columns. A slightly more complex approach is to use the
   min/max widths of unspanned cells to weight how spanned widths are
   apportioned. Experimental study suggests a blend of the two
   approaches will give good results for a wide range of tables.

   The table borders and intercell margins need to be included in
   assigning column widths. There are three cases:

   1.  The minimum table width is equal to or wider than the available
       space. In this case, assign the minimum widths and allow the
       user to scroll horizontally. For conversion to braille, it will
       be necessary to replace the cells by references to notes
       containing their full content. By convention these appear before
       the table.

   2.  The maximum table width fits within the available space. In this
       case, set the columns to their maximum widths.

   3.  The maximum width of the table is greater than the available
       space, but the minimum table width is smaller. In this case,
       find the difference between the available space and the minimum
       table width, lets call it W. Lets also call D the difference
       between maximum and minimum width of the table.

       For each column, let d be the difference between maximum and
       minimum width of that column. Now set the column's width to the
       minimum width plus d times W over D. This makes columns with
       large differences between minimum and maximum widths wider than
       columns with smaller differences.

   This assignment step is then repeated for nested tables using the
   minimum and maximum widths derived for all such tables in the first
   pass. In this case, the width of the parent (i.e. enclosing) table
   cell plays the role of the current window size in the above
   description. This process is repeated recursively for all nested
   tables. The topmost table is then rendered using the assigned widths.
   Nested tables are subsequently rendered as part of the parent table's
   cell contents.

   If the table width is specified with the WIDTH attribute, the user
   agent attempts to set column widths to match. The WIDTH attribute is
   not binding if this results in columns having less than their minimum
   (i.e. indivisible) widths.




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RFC 1942                      HTML Tables                       May 1996


   If relative widths are specified with the COL element, the algorithm
   is modified to increase column widths over the minimum width to meet
   the relative width constraints. The COL elements should be taken as
   hints only, so columns shouldn't be set to less than their minimum
   width. Similarly, columns shouldn't be made so wide that the table
   stretches well beyond the extent of the window. If a COL element
   specifies a relative width of zero, the column should always be set
   to its minimum width.

HTML Table DTD

   The DTD or document type definition provides the formal definition of
   the allowed syntax for HTML tables.

<!-- Content model entities imported from parent DTD:

  %body.content; allows table cells to contain headers, paras,
  lists, form elements and even arbitrarily nested tables.

  %text; is text characters, including character entities and
  character emphasis elements, IMG and anchors
-->

<!ENTITY % attrs
       "id      ID       #IMPLIED  -- element identifier --
        class   NAMES    #IMPLIED  -- for subclassing elements --
        lang    NAME     #IMPLIED  -- as per RFC 1766 --
        dir   (ltr|rtl)  #IMPLIED  -- I18N text direction --">

<!--
 The BORDER attribute sets the thickness of the frame around the
 table. The default units are screen pixels.

 The FRAME attribute specifies which parts of the frame around
 the table should be rendered. The values are not the same as
 CALS to avoid a name clash with the VALIGN attribute.

 The value "border" is included for backwards compatibility with
  <TABLE BORDER> which yields frame=border and border=implied
  For <TABLE BORDER=1> you get border=1 and frame=implied. In this
 case, its appropriate to treat this as frame=border for backwards
 compatibility with deployed browsers.
-->

<!ENTITY % Frame "(void|above|below|hsides|lhs|rhs|vsides|box|border)">






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RFC 1942                      HTML Tables                       May 1996


<!--
 The RULES attribute defines which rules to draw between cells:

 If RULES is absent then assume:
     "none" if BORDER is absent or BORDER=0 otherwise "all"
-->

<!ENTITY % Rules "(none | groups | rows | cols | all)">

<!-- horizontal placement of table relative to window -->
<!ENTITY % Where "(left|center|right)">

<!-- horizontal alignment attributes for cell contents -->
<!ENTITY % cell.halign
        "align  (left|center|right|justify|char) #IMPLIED
         char    CDATA   #IMPLIED -- alignment char, e.g. char=':' --
         charoff CDATA   #IMPLIED -- offset for alignment char --"
        >

<!-- vertical alignment attributes for cell contents -->
<!ENTITY % cell.valign
        "valign  (top|middle|bottom|baseline)  #IMPLIED"
        >

<!ELEMENT table - - (caption?, (col*|colgroup*), thead?, tfoot?, t
                    body+)>
<!ELEMENT caption - - (%text;)+>
<!ELEMENT thead - O (tr+)>
<!ELEMENT tfoot - O (tr+)>
<!ELEMENT tbody O O (tr+)>
<!ELEMENT colgroup - O (col*)>
<!ELEMENT col - O EMPTY>
<!ELEMENT tr - O (th|td)+>
<!ELEMENT (th|td) - O %body.content>

<!ATTLIST table                    -- table element --
        %attrs;                    -- id, lang, dir and class --
        align   %Where;  #IMPLIED  -- table position relative to --
                                   -- window --
        width   CDATA    #IMPLIED  -- table width relative to window --
        cols    NUMBER   #IMPLIED  -- used for immediate display mode --
        border  CDATA    #IMPLIED  -- controls frame width around --
                                   -- table --
        frame   %Frame;  #IMPLIED  -- which parts of table frame to --
                                   -- include --
        rules   %Rules;  #IMPLIED  -- rulings between rows and cols --
        cellspacing CDATA #IMPLIED -- spacing between cells --
        cellpadding CDATA #IMPLIED -- spacing within cells --



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RFC 1942                      HTML Tables                       May 1996


        >

<!-- ALIGN is used here for compatibility with deployed browsers -->
<!ENTITY % Caption "(top|bottom|left|right)">

<!ATTLIST caption                  -- table caption --
        %attrs;                    -- id, lang, dir and class --
        align  %Caption; #IMPLIED  -- relative to table --
        >

<!--
COLGROUP groups a set of COL elements. It allows you to group
several columns together.
-->
<!ATTLIST colgroup
        %attrs;                    -- id, lang, dir and class --
        span    NUMBER   1         -- default number of columns in --
                                   -- group