I just blew up my profile?

Hihi, I was attempting to change my profile and putting HTML code into my about me. I've done it before so I was confident, and was trying to label each section of HTML code so I was better aware of what it all did. Following advice I had found from a SpaceHey comment, I decided to put in <FlyingRainbowDash> as a hidden label (related to the piece of code), and when I hit save to test, it removed all pieces of HTML code. When trying to reformat things and do simple things such as </br>, it gets deleted every time I press save. I tried deleting everything from the about me, refreshing the page, leaving and returning to it, but nothing would fix it. What did I fuck up and how do I go about fixing it? Thanks :)