Detecting color phantoms

[dginev]

Recording a good idea from our last LaTeXML meeting for future consideration:

Recent news have spotted notable uses of white color text on white background to conceal text in arXiv. Bruce pointed out that our fine-grained use of the Font object allows us to detect these somewhat reliably, and we could mark them in some way.

I think adding a class="color_phantom" could be a good starting point - for discussion or implementation - when the foreground and background colors are at a very small distance from each other.