The decode really, really, really should not be happening client side in Javascript. The backend should handle it before handing the text to the user's browser. You are correct; If this is done client side it means a bad actor can mess with it and/or include an injection attack of some sort.

Nothing client side should ever handle user input, except perhaps convenience features like flagging incomplete fields or kicking the cursor to the next input element when one is full (e.g. for phone numbers). Anything client side can be fucked with by the client. Validation needs to happen on the server side, before committing the input to the database (or doing whatever it's going to do with it).

There are a lot of potential pitfalls any time you accept text input from a user, store it, and regurgitate it back to display on a user's browser. The thing is, HTML (and all HTML-encapsulated scripting languages) are just text. So regular words and a block of Javascript that makes dancing polka-dotted hippos dance across your screen and incessantly play the Hamster Dance song at 200% volume are, without protections, input and stored exactly the same way. Preventing ne'er-do-wells from doing injection attacks with SQL calls, HTML, control and escape characters, Javascript, etc. is part of a whole industry.

It appears lemmy does filter out raw HTML tags, at least. I tried to insert one in my last comment just for illustration and it was silently removed from the input.