For a web application, unstyled HTML is almost certainly a mess even if it's perfectly semantic, but for a web page you absolutely don't need any CSS at all for it to be at the bare minimum readable on almost any device.
Sure it's mostly black text on white backgrounds with only minor typesetting differences between elements but as a means to present information it has worked since the mid 1400s...
Sure it's mostly black text on white backgrounds with only minor typesetting differences between elements but as a means to present information it has worked since the mid 1400s...