Unfortunately, the yEd help is not correct here.
For '>' and '&' there are cases where the HTML parser knows that these characters cannot belong to a tag or an entity. E.g. if there is no '<' before '>' it cannot be a tag. Moreover, if '&' is not the first character of a word ending with ';', it cannot be an entity. If you use "a <b or c> d" as description, neither '<' nor '>' will be displayed. Spaces (or rather the missing spaces) are important here.