Checkpoints - Do not use underlining to highlight text
Underlines make text harder to read, which can be particularly problematic for people with dyslexia.
WCAG failure F2 looks into issues where the appearance of text conveys a meaning that is not using the correct markup. Users expect links to be underlined. When a user sees an underlined text they expect it to be a link, therefore one should never underline text in the online world unless a link is being presented.
While the underline <u> tag was deprecated in html 4.01, it has a few, very specific use cases in HTML 5 such as misspellings and proper nouns in Chinese. Using a <u> element to add a visual underline on a link is not a proper use case of the element in HTML 5 and as such using a deprecated tag is also a WCAG issue.
If the <u> is used to underline a link, CSS should be used instead. If a text is not a link then you may need to review why it needs to be underlined, it is suggested to use <strong> or <em> tags to add an emphasis on the text.
Accessibility (WCAG 2 Level AA, 1.3.1), Usability
This checkpoint examines the entire HTML source code of a page and reports if any underline <u> elements are used.