Properties that Apply to Column and Column-group Elements

Only a few properties can be applied to elements with a display property value of table-column or table-column-group:

• the border properties, but only in the collapsing borders model (see below)
• the background properties, where cells and rows have transparent backgrounds
• the width property
• the visibility property value collapse—any other visibility values are ignored for columns and column groups

Table Width Algorithms

Unlike other block boxes, a table with zero horizontal margins and a width property that’s set to auto doesn’t size to fill its containing block. Instead, the table size will be determined by its contents. A table can be horizontally centered by setting margin-left and margin-right to auto, though.

There are two very different algorithms for determining the widths of table columns: the fixed table layout algorithm and the automatic table layout algorithm. These are specified with the table-layout property (which takes values of fixed, for fixed layouts, and auto, for automatic layouts); its initial value is auto. If the table’s width is specified as auto, the automatic table layout algorithm is normally used. In the case of block-level tables (when display is set to table), user agents are allowed to use the fixed table layout algorithm anyway, but they aren’t required to.

With the fixed table layout algorithm, the widths of columns and of the table are not governed by the contents of the table’s cells. Instead, the width of each column is determined as follows:

• Column objects whose width is not auto set the width for that column.
• A cell in the first row, whose width is not auto, sets the width of the column it belongs to. If the cell spans more than one column, the width is divided over the columns.
• Any remaining columns equally divide the remaining horizontal space, minus any borders or cell spacing.

The width of the table is the greater of the value of the table’s width property, and the sum of the column widths (plus borders or cell spacing). If the table is wider than the columns, the extra space will be distributed over the columns.

The automatic table layout algorithm usually requires more than one pass. The CSS2.1 specification suggests an algorithm for determining column widths, but user agents are not required to use it.

The suggested algorithm for determining column widths examines every cell in the entire table, computing the minimum and maximum widths required for rendering each cell. These values are then used to determine how wide each column should be, which in turn may decide the width of the table itself.

Table Height Algorithms

If the table’s height property has a value other than auto, and the specified height differs from the sum of the row heights plus borders or cell spacing, the behavior is undefined.

Percentage values for the height property are undefined for rows, row groups, and cells.

The vertical-align property of each cell determines its alignment within the row. Only the values baseline, top, bottom, and middle are allowed. For any other value, baseline will be used.

Borders On Table Objects

There are two different models in CSS2 for rendering borders around internal table objects: the separated borders model and the collapsing borders model. We can choose the model we prefer by using the border-collapse property, and setting the value to separate (the initial value) or collapse.

In the separated borders model only cells (and the table itself) can have borders; rows, row groups, columns, and column groups cannot. Borders are drawn individually around the cells and the cells are separated by the vertical and horizontal distances specified by the border-spacing property. In the space between cell borders, the backgrounds of rows, row groups, columns, and column groups are invisible. Only the table background is visible in the inter-cell spacing. Figure 2 shows an example of a table that’s rendered using the separated borders model.

Figure 2. Rendering a table with separated borders

Here’s the relevant CSS for the table:

table {
  border-collapse: separate;
  border-spacing: 1em 0.5em;
  background-color: #ddd;
}

Another property that applies in the separated borders model is the empty-cells property. It controls whether cells that lack visible content have borders and backgrounds (if the value is show, the initial value) or not (if the value is hide). Carriage returns, line feeds, tabs, and blanks are not considered to be visible content, although a non-breaking space is.

In the collapsing borders model, the cells aren’t separated from one another and their borders—along with borders of rows, row groups, columns, column groups and the table itself—collapse (or overlap) in a rather complicated way. An example of a table to which the collapsing borders model is applied is shown in Figure 3.

Figure 3. Rendering a table with collapsed borders

With this model, quite a few borders may be specified in such a way that they would be rendered in the same place. The CSS2.1 specification provides an algorithm for border conflict resolution—that is, which border will win, or be rendered, in these situations. Very broadly speaking, the most eye-catching border will be rendered, unless at least one of the borders has border-style set to hidden, in which case no border will be rendered.

If none of the borders are hidden, wide borders win over narrow borders. If two or more borders have the same width, the border-style property decides which one will be rendered. The styles are preferred in the following order: double, solid, dashed, dotted, ridge, outset, groove, and inset. Borders with border-style set to none have the lowest priority, and will never win over other border styles—even if they have a large width value.

If there is still no winner, the algorithm looks at the objects for which the borders are set. The preferred order is: cell, row, row group, column, column group, and table.

The border-spacing and empty-cells properties are ignored when the collapsing borders model is used.