80% of PHP?

Something I see on many Developers' websites and CVs is percentages or levels of how well they know certain tools and frameworks.

Things like "80% of PHP" or "Advanced in HTML and CSS".

But how do you quantify that?

Do people alter their percentages accordingly if a new feature is added to a language or framework?

Or, instead of trying to show how much you understand, focus on what problems you can solve with those tools and how you can provide value to customers or employers instead of what tools you'd use to do it.