Semantics are non-obvious with href on a lot of elements like tr/td/form/input/textarea/button/head/script/style for example. A lot of work would go into specifying and implementing the interactions of combinations of href on table/colgroup/tbody/tr/td for example - for no significant benefit.

Older browsers might go crazy

Who cares? If you use HTML5 you have to throw them under the bus anyway.

And most (older) browsers ignore unknown attributes anyway.

HTML5 in general degrades fairly well, with the exception of with browsers which simply don't support CSS at all (e.g. IE5 and below).

You're right to say that older browsers would ignore the unknown attribute, the problem is that having links broken might degrade that site to a point where it isn't usable.

But it would be pretty easy to create a polyfill that attaches onclick handlers to the respective elements.

Actually I'm tempted to write one to get this functionality with current browsers.

[edit] As Oscar Wild put it: The only way of getting rid of a temptation is to yield to it.

https://github.com/webcrofting/href4all

That's awesome! From concept to github in a flash. Well done.

This would be a pretty easy thing to polyfill, I think.

You may want to read up on HTML Design Principles: <http://www.w3.org/TR/html-design-principles/>.