Valid XHTML alternative to

Today I noticed that a page on this site failed validation. W3C's markup validation service gave the following error:

element "strike" undefined

<strike> is not valid XHTML; I'd forgotten the correct XHTML markup for this purpose:

my favourite colour is <del>red</del> <ins>white</ins>

The above gives: my favourite colour is red white

It's a good idea to explicitly define the appearance of deleted and inserted text in your style sheet:

del { text-decoration: line-through; }
ins { text-decoration: underline; }