Web Style Sheets
CSS tips & tricks

Status

Warning: at the time of writing (February 2003), the major browsers don't yet support the 'nth-child' selector (introduced in November 2001) and only very few support the COL element.

Even and odd rules

One way to improve the readability of large tables is to color alternating rows. For example, the table below has a light gray background for the even rows and white for the odd ones. The rules for that are extremely simple:

tr:nth-child(even) {background: #CCC}
tr:nth-child(odd) {background: #FFF}

Month	1994	1995	1996	1997	1998	1999	2000	2001	2002
Jan	14	13	14	13	14	11	11	11	11
Feb	13	15	12	15	15	12	14	13	13
Mar	16	15	14	17	16	15	14	15	15
Apr	17	16	17	17	17	15	15	16	16
May	21	20	20	21	22	20	21	20	19
Jun	24	23	25	24	25	23	25	23	24
Jul	29	28	26	26	27	26	25	26	25
Aug	29	28	27	28	28	27	26	28	26
Sep	24	23	23	26	24	24	24	22	21
Oct	20	22	20	22	20	19	20	22
Nov	18	17	16	17	16	15	14	15
Dec	15	13	13	14	13	10	13	11

In fact, CSS allows not only allow even/odd alternations, but arbitrary intervals. The keywords 'even' and 'odd' are just convenient shorthands. For example, for a long list you could do this:

li:nth-child(5n+3) {font-weight: bold}

This says that every 5th list item is bold, starting with the 3rd one. In other words, the items numbered 3, 8, 13, 18, 23, etc., will be bold.

Even and odd columns

The same works for table columns, too, but then there has to be an element in the document that corresponds to the column. HTML provides COL for that. The table has to start with one COL for every column:

<table>
<col><col><col><col><col><col><col><col><col><col>
<tr><th>Month<th>1994<th>1995<th>1996...

(COL can be used for other things than style, but in this case all we need is that the COL elements are present.) The following rules give the first column a yellow background, and then every other column starting from column 3 a gray one:

col:first-child {background: #FF0}
col:nth-child(2n+3) {background: #CCC}

Month	1994	1995	1996	1997	1998	1999	2000	2001	2002
Jan	14	13	14	13	14	11	11	11	11
Feb	13	15	12	15	15	12	14	13	13
Mar	16	15	14	17	16	15	14	15	15
Apr	17	16	17	17	17	15	15	16	16
May	21	20	20	21	22	20	21	20	19
Jun	24	23	25	24	25	23	25	23	24
Jul	29	28	26	26	27	26	25	26	25
Aug	29	28	27	28	28	27	26	28	26
Sep	24	23	23	26	24	24	24	22	21
Oct	20	22	20	22	20	19	20	22
Nov	18	17	16	17	16	15	14	15
Dec	15	13	13	14	13	10	13	11

The background of rows (TR) is drawn in front of the background of columns (COL), so if you want to be sure that the background of the columns is visible, you should not set a background on the rows.