codepen.io is the suggested place, but it doesn’t have to be that.
if you agree with codepen, after you create an account look for the part of the description that talk about testing, open the given pen, and use the “Fork” button in bottom right corner to copy it to your account
in this pen there is a script tag, do not delete it.
Add your html in the html box, your css in the css box, and now and then use the test suite to check how you are doing