I want to learn js and use that for blockchain development so I was wondering if I need to learn html and css to be able to master js
Yes. You use js to modify content in Html Or web page. So you should know what and how web or html page works to master JS*. Don't bother of anything other to go with, It is easy compared to js.
I think we need to define what blockchain development is. Are we talking about creating web applications for existing blockchains or are we talking about development of a new blockchain.
If you want to learn programming for blockchain there is also "Solidity" you could learn.