HTML doesn't handle URIs any differently. They're just string attributes on elements(generally).

Sure it does, when it defines the semantics of the elements. HTML says "the 'href' attribute of the 'a' element is the URI of a linked resource". There's no special syntax, because it doesn't need to have; they're identified by their elements.

But on a more general format that doesn't define particular semantics to nodes, you have syntax that identifies URLs. For example, in Turtle - which is a format that I quite like - you encode strings using double quotes around the value, but URIs are encoded using angle brackets.

Example:

  dc:title "RDF/XML Syntax Specification (Revised)" ;
  ex:editor [
    ex:fullname "Dave Beckett";
    ex:homePage <http://purl.org/net/dajobe/>
  ] .

The advantage is that smart clients can parse that information and crawl the documents, generating useful data even if they don't necessarily understand this particular format. Think search engines, for example - they might not understand that <span id="author"> identifies the author's name, but they can still derive useful information for the web of pages that they crawl.