Re: Why is querySelector much slower?

[Boris Zbarsky]

On 4/28/15 1:58 AM, Glen Huang wrote:
Just a quick follow up question to quench my curiosity: if I do "list[1]" and no one has 
ever asked the list for any element, Gecko will find the first two matching elements, and store 
them in the list, if I then immediately do "list[0]", the first element is returned 
without walking the DOM (assuming there are at least two matching elements)?

Yes, exactly.

querySelector("foo") and getElementsByTagName("foo")[0] can return different 
nodes

Still a bit confused regarding this. If the premise is the selector only 
contains characters allowed in a tag name

Then in that case I _think_ those are now equivalent, though I'd have to 
check it carefully.  They didn't use to be back when 
getElementsByTagName was defined as matching on qualified name, not 
localname...

-Boris