Just like it previously was the case with education.minecraft.net (WEB-934), the SSL certificate for skins.minecraft.net ran out on November 28, 2017.
[media]
Example link: https://skins.minecraft.net/MinecraftSkins/violine1101.png
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Yes, that's what WEB-985 is about. I left the report open because even though the api is no longer working, the website still should have a valid certificate.
MojangSupport tweeted that skins.minecraft.net is retired: https://twitter.com/MojangSupport/status/988654862945533952
I'm guessing it isn't deserving of a valid SSL Certificate.
Also if you check out other related threads you'll see other Mojang representatives have said the same. http://wiki.vg/ may be the place to visit for this documentation.