Web page looks different in every browser - What can I do?

HTML & CSS
5

My pages are on a WAMP server at home. Not published yet

Chrome : 91.0.4472.124
Safari : 5.1.7 (7534.57.2)
Edge : 91.0.864.67
Explorer : 11.0.9660.19596 update 11.0.170
Firefox : 90.0b12
Opera : 77.0.4054.203