From the parsing algorithm:
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If buffer is ".." and c is one of EOF code point, "/", and "\", set the last string in url's path to the empty string.

Otherwise, if buffer is "..", remove the last string from url's path.
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For URLs like:  

http://example.com/../../filename  ,

when there are no strings in the path yet, after the first "../" is read,
it's undefined what happens. Does it trigger an error?  
Does the parser return failure?  Or is that step just simply ignored? 

This may also affect this rule, found elsewhere in the algorithm:
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Set url's host to base's host, url's port to base's port, url's path to base's path, then remove url's path's last string, set state to relative path state, and decrease pointer by one.
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