Tim is right that in general, strings should ideally contain no HTML. Especially if that HTML is then bounded to CSS styling. You should not rely on other language packs following the HTML structure present in the English string.
Richard is right that almost every theme has massive HTML formatting in its chooseme string. I do not know who started with it but it has spread like a virus However, I can understand that it is useful there (although I would prefer seeing Markdown there, like we have in help strings).
Similarly I can understand the need for nicely formatted welcome emails. If it is something you do via the local language customization, then feel free to do what fits your needs (as it does not affect Moodle translators). If it is a plugin and you plan to share it, then I would recommend to keep the default English as plain as possible - as it can be always locally customized.