tutorial

https://mdbootstrap.com/docs/jquery/utilities/borders/

https://www.w3schools.com/bootstrap4/bootstrap_ref_all_classes.asp

https://getbootstrap.com/docs/4.5/getting-started/browsers-devices/#supported-browsers

https://getbootstrap.com/docs/4.0/components/card/

ng bootstrap

https://github.com/ng-bootstrap/ng-bootstrap

awesome bootstrap

https://github.com/therebelrobot/awesome-bootstrap

cheat sheet

https://hackerthemes.com/bootstrap-cheatsheet/

https://websitesetup.org/bootstrap-cheat-sheet/

https://bootstrapcreative.com/resources/bootstrap-4-css-classes-index/

https://www.creative-tim.com/cheatsheet/bootstrap4