As someone who does a fair share of browsing on Tor Browser's Safest mode (JS disabled) i entirely disagree with this point. Most use of JS makes my (old) computer really slow and usually leaks memory in some way. This is true on Firefox, Chromium, and all derivatives.

Yes the performance impact of a single "alert()" when i open the page is negligible. The performance impact of parsing Markdown and rendering it as HTML nodes is definitely non-negligible.

I love the idea to support more markdown formats natively for the web (eg. markdown), but i hate client-side scripting shenanigans that don't respect me as a user without the fancy latest iphone. If only some browsers were concerned about emerging protocols/formats & UX instead of increasingly-useless (except for tracking purposes) JS APIs...