<p>Recently I've been seeing a lot of this:</p>

<pre><code>&lt;a href='http://widget-site-example.com/example.html'&gt;
    &lt;img src='http://widget-site-example.com/ross.jpg' alt='Ross&amp;#39;s Widget' /&gt;
&lt;/a&gt;
</code></pre>

<p>Is it valid to use single quotes in HTML? As I've highlighted above it's also problematic because you have to escape apostrophes.</p>

It's certainly valid to use single quotes (
HTML 4.01, section 3.2.2
). I haven't noticed such a trend, but perhaps there's some framework that powers web sites you've visited that happens to quote using single quotes.