What this does
Removes the whitespace between tags that only exists for layout, plus comments unless you keep them. Text content, attributes, entities, CDATA, and the DOCTYPE are not changed.
Example
An indented document of a few dozen lines becomes one line such as <a><b>1</b></a>. Spaces inside text are preserved, so Hello <b>big</b> world keeps its spaces.
Safety
The input is validated first, and anything marked xml:space="preserve" is copied verbatim. Applications that treat whitespace-only text as meaningful, rare outside mixed-content documents, should be checked after minifying.
Privacy
Everything runs in your browser. Your input is never uploaded, stored, or put in the URL.