[whatwg] attribute value normalization

[Christoph Päper]

Anne van Kesteren:
> It should basically not happen at all. It appears that no browser has
> implemented the 'may' from HTML4

That's the logic of tagsoup parsers, i.e. their inconsistency regarding 
authoring sloppiness. If it had been "should not", they would probably 
do it. ;)

Did you also try (true) XHTML1?

> and therefore we can now say browsers must not trim.

IMO trimming is still desired behavior (and fairly easy to do), 
therefore "must not" is way too strong.