You can see browser compatibility for ES6 here
https://kangax.github.io/compat-table/es6/
Make sure to have the “show obsolete platforms” checked. You can see that IE has very poor support for updated JS syntax and features.
A few solutions for this are to use transpilers like Babel to convert new code features to basically look like and work like old code.
That and the use of poly fills.
CSS has a lot of new features that also aren’t supported on all old browsers. It’s a challenge to be sure when writing good css to make. Using something like Bootstrap helps a lot because they’ve done all the hard work of compatibility for you! But keep an eye out. Latest bootstrap dropped support for some older browser versions.