@kwxza, browsers are lenient and do their best to render pages despite a failure to compliance.
The issue comes into play when they (browsers) do adhere to a rule that you’re overlooking today or you work for a company that wants you to be compliant now because they don’t want to spend the money in the future to have you go back and rewrite your code.
For example, you wrote this and it works and you got paid for it. Six months browsers become compliant and your code doesn’t work. The company doesn’t want to pay you for something that should work and you don’t want to have to spend time to fix something that previously worked even though it wasn’t compliant.
Code validators (HTML, CSS, JS, etc) are there to help you as a developer. You’re not expected to know all the rules but you should know how to use tools that help you to know that what works today will also work tomorrow.