How do I know if a CSS property is fully supported across browsers?

I love this question! There is a tool that does exactly that. It’s called caniuse.com. This site will let you know if a certain HTML element, CSS property, etc. is supported on major browsers.

For example, take a look at the support for CSS Grid.

The dark green means the browser will display the property exactly as intended. The light green means you may have some issues displaying the property in that particular browser (and the issues will be listed below). The red means it is a no-go, and that property will not work.

One possible way to solve issues in the light green (possible problem) areas is to use a prefix. You can use a tool like Autoprefixer or Should I Prefix to see if there are any prefixes available for your particular CSS property, that will make the property work in problem browsers.