Is it correct to use single quotes for HTML attributes?

Recently I've been seeing a lot of this:

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

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

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.

I find using single quotes is handy when dynamically generating HTML using a programming language that uses double quote string literals.

e.g.

String.Format("<a href='{0}'>{1}</a>", Url, Desc)

When using PHP to generate HTML it can be easier to do something like:

$html = "<img src='$url' />";

than concatenating a string with a variable with a string, as PHP parses variables in double-quoted strings.

Someone may use it in PHP to avoid escaping " if they're using double quoted string to parse variables within it, or to avoid using string concatenation operator.

Example:

echo "<input type='text' value='$data'/>";

instead of

echo "<input type=\"text\" value=\"$data\" />";

or

echo '<input type="text" value="' . $data . '" />';

Nowadays I always stick to using double quotes for HTML and single quotes for Javascript.