I think it might make more sense to use WebAssembly and make them as extensions which are included by default (many other things possibly should also be made as extensions rather than built-in functions). The same can be done for picture formats, etc. This would improve security while also improving the versatility (since you can replace parts of things), if the extension mechanism would have these capabilities.

(However, I also think that generally you should not require too many features, if it can be avoided, whether those features are JavaScripts, TLS, WebAssembly, CSS, and XSLT. However, they can be useful in many circumstances despite that.)