fix for anonymous block boxes
remove block exception
remove unused import
add comment
fix comment
auto merge of #1461 : ksh8281/servo/block_black_boxes, r=SimonSapin

fix for anonymous boxes.
http://www.w3.org/TR/CSS2/visuren.html#anonymous-block-level
fix try_to_append_line for ImageBox
auto merge of #1463 : ksh8281/servo/img_new_line, r=pcwalton

````
<div>
   test
   <img src="test.jpeg"/>
</div>
````

if reduce window size, img is disapper.
this pr fixes this case.
Remove needless code.

This `OK(())` should be propagated from `Element.set_attribute()`.
Split up the utility function to make internal attribute's namespace.
Enable only remove operation for Document::update_idmap().
Implement Element.removeAttribute()/removeAttributeNS().