HTML has a presentational element that was originally used to underline text; this usage was deprecated in HTML4 in favor of the CSS style {text-decoration: underline}. In HTML5, the tag reappeared but its meaning was changed significantly: it now "represents a span of inline text which should be rendered in a way that indicates that it has a non-textual annotation". This facility is intended for example to provide a red wavy line underline to flag spelling errors at input time but which are not to be embedded in any stored file (unlike an emphasis mark, which would be).
