For a free CSS framwork - Bootstrap is great. w3schools extremely helpful for tips on how to use Bootstrap. You can really use w3schools for everything.
To inlcude Bootstrap, it is easiest to use a CDN and you can find the code for the CSS and JS here: Bootstrap 5.1 or here: w3schools.
| Type | URL |
|---|---|
| Bootstrap | Bootstrap 3.3.7 |
| w3schools | w3schools |