Is a shovel a must when you want to build now adays?
Well, it depends on the job. Probably not for a treehouse, but definitely for installing underground wiring and pipes.
It's the same way with languages. I laugh when I hear people asking about "What's the best language?" There really should be no plan to how you learn languages other than: What task needs to get done, and which language has the best support to solve that task?" The only exception to this in my book is when someone is so new that they just pick whatever language they can understand or learn at the time... This is the initial hurdle phase and in this instance, by all means, pick your "favorite" one.
For web development, JavaScript is a must. This is a fact right now. If you're a back-end developer and you know no JavaScript, I don't want to work anywhere near you because that means you don't care enough about your job to educate yourself on the tools necessary.
If your a front-end developer, well, I'm not even going to address that because that's obvious.
I will say though that people are often surprised at what can be accomplished with just HTML/CSS at times. I remember I made an entire form validation system using just CSS, and it worked like a charm (ofc I had a back-end one too, but no JS).
I'm not a JS fanboy at all, and in fact, I feel that the JS community is lacking in a lot of areas because from my experience many people who I've known who knew JS seemed to worship it... And generally, when someone worships their tool(s), that's a bad sign.
That said, JS is definitely a must if you're doing web, hands down. If you don't know this, Go get your hands dirty for a while and you'll soon see what I mean. Even PHP applications, Wordpress, C#, etc... All use JS.