There are many problems with that though. For example, if your CSS changes you change the filename... but now you need to change the HTML file that references i

There are many problems with that though. For example, if your CSS changes you change the filename... but now you need to change the HTML file that references it. You can't change that easily. Or what if your CSS change just deletes some unused classes... it'd be fine for users to keep the old version until it expires. If you rename the resource you'll be causing a lot of users to wait unnecessarily. Not a huge problem, unless you're Meta or Google. And so on. When people say cache invalidation is hard it's best to believe them, because it is.