Web-accessible files, like JavaScript, CSS, and images go in this
directory. You can reference them directly in your HTML. For example,
if you have a file called "style.css" and your extension is called
"Foo", you would put it in "extensions/Foo/web/style.css", and then
you could link to it in HTML like:

<link href="extensions/Foo/web/style.css" rel="stylesheet" type="text/css">