HTML

HTML is the code behind every web page and most formatted emails. It wraps text in tags that say what each piece is, a heading, a paragraph, a link, and it can carry styling instructions that decide size, colour and spacing. Your browser or mail app reads the tags and paints the result. You never see the tags, only the outcome.

That gap between the code and what you see matters for AI. Text can be styled to zero size or coloured white on white, invisible to the reader while still fully present in the underlying HTML. When an AI tool is fed the raw HTML rather than the visible text, it reads everything, including whatever was hidden there deliberately. That is the basis of several documented prompt injection attacks on email summarizers.