Got around to validating this blog with the W3C Validator, and discovered there were 19 errors involving the category links.
Apparently, WordPress sets up these links with a rel="category tag" attribute, which doesn’t pass validation.
Ispire.me has the fix for this error, which is repeated here for my own future reference:
- Go to the /wordpress/wp-includes/ directory.
- Find the category-template.php file. Make a copy, and call it category-template-original.php.
- Edit the category-template.php file with Notepad++.
- Find line 163, which reads:
$rel = ( is_object( $wp_rewrite ) && $wp_rewrite->using_permalinks() ) ? 'rel="category tag"' : 'rel="category"';
- Edit the part of the line which says:
rel="category tag"torel="tag". The line should now read†:$rel = ( is_object( $wp_rewrite ) && $wp_rewrite->using_permalinks() ) ? 'rel="tag"' : 'rel="category"';
- Save category-template.php and upload it to /wordpress/wp-includes/.
- Repeat steps 1-6 each time WordPress version updates are installed.
† The code in Step 5 is word-wrapped. If copying & pasting, do NOT add returns to this code!