Markup: Consider having a whitelist of CSS classes

I can understand the reasoning behind filtering out class attributes. Rather than strip them entirely, could we have a whitelist of classes instead?

I just finished writing this on another issue, so I'm just going to copy+paste my own words:

I feel GitHub should have a whitelist of CSS classes to apply basic inline formatting. This would permit users to use <pre><code> … </code></pre> blocks to establish the same look as fenced code-blocks, but have proper control over the syntax highlighting.

E.g., this:

# Ignore the fact this is Perl, and pretend it's some unheard-of language
our $site = "needs this";

... might be implemented like this:

<pre><code>
<span class="code-comment"># Ignore the fact this is Perl, and pretend it's some unheard-of language</span>
<span class="code-keyword">our</span> <span class="code-variable">$site</span> = <span class="code-string">"needs this"</span>;
</code></pre>

Yes, this is cumbersome to write. But for authors who're serious about having code presented readably and cleanly, it's better than absolutely nothing.

This would also allow true diff-style highlighting within GitHub-flavoured Markdown (green and red backgrounds, etc).

Some possible suggestions:

code-keyword
code-number
code-string
code-modifier
code-constant
…
diff-changed-line  (light green)
diff-changed       (darker green for parts that actually changed)
diff-removed-line
diff-removed