Elements in lxml have a sourceline attribute, but it is always None in trees parsed with html5lib.

html5lib also appears to keep track of source line numbers for error messages.

I’d like to connect the two so that WeasyPrint can do its own error reporting based on line numbers of HTML elements. It seems like the attribute should be set in etree_lxml.py, but I don’t know how to access the input stream from there.

Suggestions?