<img> (src attribute)
<audio> (src attribute)
<video> (src attribute)
<object> subresources (when an <object> performs HTTP requests)
<script> (src attribute)
<link> (href attribute) (this includes CSS stylesheets)
<iframe> (src attribute)
XMLHttpRequest requests
All cases in CSS where a url value is used (@font-face, cursor, background-image, and so forth).
<object> (data attribute)