It occurs to me, I -have- seen custom HTML tags. Zendesk's knowledge base software uses them now; the WYSIWYG editor inserts them to power some of its functionality, but since they're unstyled containers they don't affect the published page.
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I understand about 50% of what you're saying, so I'll take your word for it.
I could explain it in detail with images and code snippets and stuff but I'd need some sort of dedicated web space for that