How do you measure knowledge?

Question

I keep seeing portfolios with a skill section and with values as high as 95 but how do you know you are at x number in x language . Is it based off the number of certifcates? The number of projects you've done? Or is it a you thing where you decide and it isnt based off of anything?

Basically, is there a standard for measuring how much experience/knowledge you have with X language to be able to say I know 95% of HTML, 80% of CSS and 99% Javascript?

Answer 1

I think those certificates don't actually tell anything.

But "what you can do" with the language will tell a lot.