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Table Backgrounds

From: fantasai <fantasai@escape.com>
Date: Sat, 11 May 2002 16:19:24 -0400
Message-ID: <3CDD7CCC.1AC14609@escape.com>
To: www-style@w3.org
CC: css2-editors@w3.org


If implementation becomes an issue, let me know; I'll write a prototype
so discussion can be based on something more concrete than mere


Text reproduced below for discussion purposes:

      ---=======Ambiguities in CSS2 Table Backgrounds=======---

                                                   Written by fantasai
                                       Sanity checking by Bernd Mielke
                                   Wording help from Christopher Hoess

    Chapter 17 of the CSS2 specification defines CSS tables and how
various properties affect table elements. In describing the effects of
the background properties, however, the spec is very ambiguous, making
it impossible to write a defensibly correct implementation. The
outstanding problems are explained below, using row backgrounds as an

-Backgrounds within border-spacing-

   | In this model, each cell has an individual border. The 'border-
   | spacing' property specifies the distance between the borders of
   | adjacent cells. /This space is filled with the background of the
   | table element./

      This raises the question of what happens to a row background
  between cells. Is it rendered invisible over the spacing or does the
  background strip split at this point, resuming its pattern in the
  next cell?

  The two concepts render as follows:


      Rendered as if the cell background were laid out with collapsed
    spacing and then the cells explode outward to their positions.

    Concept Rendering
    Final Rendering


      Rendered as if the cells are windows looking onto a view of the
    row background.

    Concept Rendering
    Final Rendering

      Both methods work fine if the table is a strict grid. However, in
  the explode model, cells spanning more than one column throw off
  background alignment across rows. This shortcoming appears in the
  diagram above. Since the window model does not suffer from this
  problem, it is superior.

-Accounting for spanning cells-

      From what I gather, the assumption seems to be that row and
  column backgrounds must be strictly rectangular. There is nothing in
  the spec that either supports or refutes this. Assuming that row and
  column boxes must be rectangular, one might have a table that looks
  like this:

  Concept Rendering
  Final Rendering

      Having a background boundary cut across at an arbitrary point
  without consideration for the cell's contents looks bad. However, the
  problem is not merely aesthetic, but also very practical: the
  contrast between text and background is compromised, and the author
  has no control over this. It is, of course, possible to explicitly
  place the background on colspanned cells, but this breaks the
  continuity of image backgrounds, which is especially noticeable with
  large patterns, and cannot emulate the layering effects possible with
  transparent background images.

      One way to avoid the contrast problem is to have the background
  of the cell's originating row cover the cell's entire background
  area, thus:

  Concept Rendering
  Final Rendering

      Either way, the spec needs clarification.

-Background Boundaries-

      The strangely specified bounds of table element backgrounds have
  been commented on before by David Baron, and later brought up by
  myself. For table rows, the spec says that the rows together cover
  the entire table. Does this include the space between the table
  border and the cell borders? Should it include the space between the
  table border and the cell borders? What about the space between
  adjacent rows?

      The problems with starting the background area in the spacing
  beyond cell borders is that the author cannot place images so that
  they line up with the edges of the visible background. Consider this

  Concept Rendering
  Final Rendering

      The image appears properly lined up with the cells', and thus the
  rows' visible, boundaries. If the background area began in the
  cellspacing, the image's border effect would be lost.

-Empty Cells-

      Tim Bagot commented on the somewhat peculiar behavior of "empty-
  cells: hide" several years ago. It clears the border and background
  of the table cell to "hide" it, but the table cell is very visible
  (though without a border) if the background is assigned on a row,
  column, row group, or column group:

  empty-cells: show
  empty-cells: hide

      Judging from the errata and traditional browsers' behavior, the
  effect indended seems to be more like:

  Table with hidden empty cell

      It certainly looks more self-consistent that way, and if one
  wanted the effect currently specified, it's easily accomplished by

  	td:empty, th:empty {
  		border: none;
  		background: transparent;


      To resolve the ambiguity with respect to table backgrounds
  defined by the CSS2 specification, I propose the following changes:

  ~Section 17.5.1~

   |   2. The next layer contains the column groups. [-The-columns-----
   |      groups-are-as-tall-as-the-table,-but-they-need-not-cover-the-
   |      whole-table-horizontally.-] >>Each column group extends from
   |      the top of the cells in the top row to the bottom of the
   |      cells on the bottom row and from the left edge of its
   |      leftmost column to the right edge of its rightmost column.
   |      The background extends to cover the full area of all cells
   |      that originate in the column group, but this extension does
   |      not affect background image positioning.<<
   |   3. On top of the column groups are the areas representing the
   |      column boxes. [-Like-column-groups,-columns-are-as-tall-as---
   |      the-table,-but-need-not-cover-the-whole-table-horizontally.-]
   |      >>Each column is as tall as the column groups and as wide as
   |      a normal (single-column-spanning) cell in the column. The
   |      background extends to cover the full area of all cells that
   |      originate in the column, but this this extension does not
   |      affect background image positioning.<<
   |   4. Next is the layer containing the row groups. [-Each-row-group
   |      is-as-wide-as-the-table.-Together,-the-row-groups-completely
   |      cover-the-table-from-top-to-bottom.-] >> Each row group
   |      extends from the top left corner of its topmost cell in the
   |      first column to the bottom right corner of its bottommost
   |      cell in the last column.<<
   |   5. The next to last layer contains the rows. [-The-rows-also----
   |      cover-the-whole-table.-] >>Each row is as wide as the row
   |      groups and as tall as a normal (single-row-spanning) cell in
   |      the row. As with columns, the background extends to cover the
   |      full area of all cells that originate in the row, but this
   |      this extension does not affect background image
   |      positioning.<< 

  ~Section 17.6.1~

   | In this model, each cell has an individual border. The 'border-
   | spacing' property specifies the distance between the borders of
   | adjacent cells. [-This-space-is-filled-with-the-background-of-the
   | table-element.-] >>In this space, the row, column, row group, and
   | column group backgrounds are invisible, allowing the table
   | background to show through.<<

      To improve the behavior of 'empty-cells', I suggest the following

  ~Section 17.5.1~

   | >>If the value of their 'empty-cells' property is 'hide'<<
   | [-T-]>>t<<hese "empty" cells are transparent >>through the cell, 
   | row, row group, column, and column group backgrounds, letting the
   | table background show through<< [-if-the-value-of-their-'empty----
   | cells'-property-is-'hide'.-]

-See Also-

  * Bos, Bert. "Re: CSS2 Table Model" (Jul 26, 1999) -
     "completely cover the table"
  * fantasai. "Backgrounds - again" (Nov 26 2001) -
     table background boundaries
  * Bagot, Tim. "Re: Table Backgrounds" (Jun 29, 2001) - 
     explode theory
  * Bagot, Tim. "Re: table backgrounds" (Jun 05, 1999) -
     empty cells
Received on Saturday, 11 May 2002 16:15:57 UTC

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