Update to web resource embedding test page

Yesterday I completely rewrote my resource embedding test to cover more cases (especially ones new in HTML5) and be more usable. In the likely event you're not familiar with it, it's a HTML document which embeds many types of resources (images, audio, other HTML, etc.) using all of the possible containers (<img>, <iframe>, etc.) to see how browsers react.

The results can be quite interesting; for example, with the current expansion I discovered that JavaScript embedded in a SVG document will not execute if embedded using <img>, but will with <object>. (In hindsight, this makes perfect sense given the fundamental design principle of Web security, namely “don't add anything which would obviously make existing sites' security assumptions false”, the assumption here being that it's safe to allow <img>s as user-generated content.)

Specific new features:

• Audio, HTML, and plain text content. (Unfortunately, some combinations cause the audio to autoplay; I tried to make it quiet and plain to make up for that.)
• <audio> and <video> containers.
• Scripts inside HTML and SVG content, which also attempt to modify window.top.
• Fixed-scrolling headers so you don't need a large window to make sense of the large table.

Let me know if you've thought of any additional useful cases.