How to find “invalid HTML” in message

Hi, I’m trying to post an other message here, but when I hit “post” red text appears, telling me my message includes “invalid HTML”. I suppose my message has some characteres which aren’t allowed to post?

The red text also says that this “invalid HTML” is highlighted, which is weird, because when I look over my message, I can’t see any highlighting.

Can someone explain to me how to find this “invalid HTML” in my message, or tell me what characters which I’m not allowed to use?