According the katex instruction, I ched if the styleshhet was loaded or not. It wasn't.
Now I am searching for the katex stylesheet to implement into the style.css of angular.
https://katex.org/docs/browser has a <link rel="stylesheet" …> in its starter template. While you may be able to download the file from its href attribute and copy its contents into your style.css, in most situations it would be preferable to just get that tag itself copied to the index.html of your Angular project. If you copy the CSS content instead of the link tag, make sure to also get all the files that the stylesheet references, fonts in particular.