How to correctly render html letters in html page?

Question

HTML

It is necessary to display an html letter (or even an entire email thread) in an html page so that the layout of the entire page does not break. Often, letter styles or unclosed tags affect or break the layout, you have to solve each case separately and write a crutch for a specific type of letter, but new cases and crooked letters appear every day. Web mail (for example, Yandex, Google, Mail) cope with this in 100% of cases.

Perhaps there is a ready-made library / service that can be fed RAW letters and it will produce an encapsulated layout?