Are you looking for a textual UML tool (https://modeling-languages.com/text-uml-tools-complete-list/) to "write" your UML models and then easily render them in your browser?
Or for a graphical online UML editor (https://modeling-languages.com/web-based-modeling-tools-uml-...)?
If the former, then plantUML is my favourite. If the latter, there quite a few options but one that's easy and fast is http://www.umletino.com/