Support "img" tag "src" attribute

From Mozilla's documentation [1]

> If the element is an […] img […] element
> The value is the resulting URL string that results from parsing the value of the element's src attribute

[1] https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/itemprop#Values

Differential Revision: https://phabricator.kde.org/D27720