<u>
A non-textual annotation — the rare legitimate underline: misspelling marks, Chinese proper-name marks.
Baseline: Widely available · Inline text semantics
When to use it
Almost never; its one idiomatic modern use is the proper-name mark in Chinese text.
When not to
Never underline links' lookalikes with it, and never use it for emphasis — readers assume underlines are links.
Example
北京是中国的首都。
<p lang='zh-Hans'><u>北京</u>是中国的首都。</p>