[whatwg] Fallback behavior

[Maciej Stachowiak]

As far as I can tell, the current spec does not adequately define how  
fallback behavior works. Specifically, what should be done with  
fallback content when not falling back?

Presumably it should be parsed into the DOM, but should not render -  
that's the de facto behavior. But I don't think the spec says that  
anywhere. Then there are weirder cases, where some element has a side  
effect other than rendering.

- should scripts in fallback content execute?
- should style elements in fallback content apply style?

There's probably others. The following test case gives different  
results between trunk WebKit and Opera on the one hand, and Firefox  
on the other hand. Although in Safari's case it is probably because  
we are throwing out the content of the canvas rather than parsing but  
not rendering.


<canvas>
<b>fallback</b>
<script>
alert('x');
</script>
</canvas>

<object data="about:blank">
<b>fallback</b>
<script>
alert('y');
</script>
</object>