2.5.14 base.text.u Underlined text
To underline text use the <u> element. Not recommended since underlines are
used by web browsers to style hyperlinks.
Example
This line of text will render as underlined
Markup: base/text/text-u.twig
<span class="xml"><span class="hljs-tag"><<span class="hljs-name">u</span> <span class="hljs-attr">class</span>=<span class="hljs-string">"</span></span></span><span class="hljs-template-variable">{{modifier_class}}</span><span class="xml"><span class="hljs-tag"><span class="hljs-string">"</span>></span>This line of text will render as underlined<span class="hljs-tag"></<span class="hljs-name">u</span>></span>
</span>
Source:
base/text/_text.scss, line 238