Table border color in CSS with border collapse

问题

I want to put a line above some field in a table, to indicate that it is a sum of the above values. However, the table already has borders by default.

Here is an example: I have a table with borders collapsed. I set the border-bottom on one field, and the border-top on the field below it. Both of these specify the same border. The CSS for the top one is used. Is there a way to use the bottom one?

<html>
    <head>
        <style type="text/css">
            table { border-collapse: collapse; }
            td.first { border-bottom: solid red 1px; }
            td.second { border-top: solid gold 1px; }
        </style>

    <body>
        <table>
            <tr><td class="first">Hello</td></tr>
            <tr><td class="second">World</td></tr>
        </table>
    </body>
</html>

This shows two cells with a red line between them. Is there way to get a gold line?

回答1:

This is a defined behavior of border-collapse. Page 357 of the O'Reilly CSS Definitive Guide 3rd Edition says:

if collapsing borders have the same style and width, but differ in color, then ... from most to least preferred: cell, row, row group, column, column group, table.

if ... come from same type of element, such as two rows... then color is taken from borders that are the topmost and the leftmost.

So the topmost, which is red, wins out.

One way to override this may be to use cell for the color to win over the color for the row.

example: http://jsfiddle.net/Chapm/

The rules that have higher precedence than this "same color rule is"

wider border wins over narrower ones

and after that,

double wins over solid, then dashed, dotted, ridge, outset, groove, inset

You can use 2px for the gold for it to win over, and I tried in Chrome to use 1px but double, and it appears as 1px solid and it will win over the red as well. Although if you want to use this method, then it may be best to make sure the browsers you support behave the same using this technique.

http://jsfiddle.net/Chapm/2/

回答2:

Just change border-collapse to separate and set border-spacing to zero.

Note: IE8 supports the border-spacing property only if a !DOCTYPE is specified.

<!DOCTYPE html>
<html>

<head>
  <style type="text/css">
    table {
      border-collapse: separate;
      border-spacing: 0px;
    }
    
    td.first {
      border-bottom: solid red 1px;
    }
    
    td.second {
      border-top: solid gold 1px;
    }
  </style>
</head>

<body>
  <table>
    <tr>
      <td class="first">Hello</td>
    </tr>
    <tr>
      <td class="second">World</td>
    </tr>
  </table>
</body>

</html>

Tested on win7 with: Chrome 16, IE 9, FF 9, Safari 5.0.5.