parse loc element_name attributes evaluates to a list of labeled parse trees, each representing an attribute argument to the element function for element_name. For example, if called on the HTML element <img src='foo' alt='bar' id='some-image'>, this function will evaluate to parse trees for the arguments:

~src:(return "foo") ~alt:(return "bar") ~a:[id (return "some-image")]

This satisfies the attribute arguments in the signature of Html_sigs.T.img.