Coding for different browsers.
Browsers fetch HTML from the server, loads all the CSS, JS, images, and other assets required and renders the page.
Rendering Engines
Browsers are the applications we use, and under the hood they use a rendering engine to interpret the code and paint the page.
Firefox - Gecko
Safari - Webkit
Chrome, Edge - Blink
Discrepancies in how engines handle certain code leads to rendering things differently and sometimes breaking entirely.
This used to be a major problem, but Microsoft adopted the engine Chrome uses so now they work the same.
CSS Compatibility
CSS specifications are actively being developed. As they are released, rendering engines need to implement them and browsers need to get the updates.
You can look up CSS, JS, and other browser API compatibility at https://caniuse.com/.