The standard NodeElement functions for getAttribute, getTagName,
getParent, and friends go back to WebDriver and parse the DOM for each
request. This is insanely slow per request, and in the case of forms we
do a lot of checking to determine the field type.
This change modifies the form field detection to copy the entire node
content into a DOMDocument and parse the document locally.
This is significantly faster - in some cases where there are large
documents minutes faster.
I believe that this should be a safe change as the document fetched from
the browser is normalised to match the doctype specified.
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