Jon Postel summed up contravariance by thinking about HTML: "Be liberal in what you accept, and conservative in what you send."
Did he say this about HTML? The original quote predates the Web. I think Postel was originally talking about subtle variations in lower-level, machine-generated, protocols, and I doubt he would have supported the mess that happened with excessively forgiving Web browsers. Mosaic was OK, but then HTML handling started getting crazy with Navigator. As the mess started, nobody put a bad-HTML indicator in the status bar, nor refused to display a page that had errors, so we had years of mess, and, even today, erroneous HTML is the norm.
This doesn't matter as much now, now that the big huge browsers have converged on HTML quirks handling, but then we went and did sloppy again with JavaScript, only much worse... Call a function with the wrong number of arguments: no problem! Wrong type: no problem! Access an undefined variable: no problem! Have the wrong object somewhere and access a slot that doesn't exist: oh baby, you know that ain't a problem! Fear does not exist in this dojo, and runtime errors are contrary to a can-do attitude! Identifiers, coming out of nowhere! Libraries full of caveats, contortions, and erroneously-handled cases that would embarrass an assembly language programmer! Glorious! Now let's do real app development, including large framework composition, with this thing!
The next time you hear an industry programmer assert that Lisps don't have static checking or are effectively write-only languages, and it turns out that said programmer does use JavaScript-- you should suggest a friendly wager of $20 or more, before your counter-argument.
(I have done real work in JS, most recently, a cool browser-based animated technical visualization part of a large, mostly Racket-based, system. Even with mitigating JS pitfalls with disciplined coding practices, the JS was a pain in the butt, compared to had I been able to use Racket for that part.)
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