The main reason to use it was the `rel` attribute for previous/next
pages not being indexed correctly by certain search engines when using a
relative URL. However, AFAIK that only applied to `<link>` tags, not to
`<a>` tags, and only if a `<base>` tag was defined.
In any case, it looks like the same search engines don't use the `rel`
attribute for previous/next to index pages anymore.
The difference is `html_safe` allows every HTML tag, including the
`<script>` tag, while `sanitize` only allows tags which are considered
safe. In this case, we want to allow a `<span>` tag in a translation,
and links inside flash messages.
