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In PHP you may mix HTML and PHP code with <?php and ?> but that is not possible in Hack. If you want mix HTML you have to use something called XHP. That is basically XML that are treated like objects.

With XHP you will get cleaner code, automatic variable escaping, XSS protection and can even get it to work with the type checker! You could write a function that will return a HTML list and you may type check that. You have to annotate the function return type like function foobar(): :li {}

If you want to read more about XHP check out this blog post by Facebook or the manual.