Do I need normalize.css when developing my own WordPress-theme?

Web standards are pretty good these days and IMO I don’t think things like normalize are nearly as necessary as years ago.

I would recommend having a look through the CSS file and deciding for yourself.

You can always just grab what styles you think are relevant and include them in your own CSS. This will cut down on a dependency and make you more aware of what code you are including in your own theme.

Quick note: You can use caniuse.com to look up specific styles or tags you might have questions about.

For Example, normalize includes this CSS…

main {
   display: block;
}

If you look up main HERE, you will see it was added to the spec late and could cause bugs in IE if you don’t apply block to it.