There are several bugs such as Bug 28300 that ask for (limited, specific) inline html elements being allowed in messages of the user interface.
A recurring example are untranslated english technical terms appearing embedded in native sentences. It is good html, good for search engines and other text analysing tools, good for speech generators and screen reading devices, when those terms are properly tagged as English. This requires an inline element such as <span lang="en"> or similar, wrapped around them. It is usually not posing a problem when it is allowed in normal messages rendered as visible html text.
For messages used in contexts where html elements cannot be used, see bug 28919