According to
https://www.w3schools.com/tags/tag_u.asp the u tag "Defines text that should be stylistically different from normal text" although they then proceed to assume it will underline the text. Some sites even said that specific styling like underlining isn't implied or required and you need to set the style in the CSS. So as long as the ereader honors the styling for the u tag in your CSS you're good to go.