Today, CSS doesn’t really have any competitors. We might compile another language to get it, or apply it in unusual ways, but ultimately styling happens on the web through CSS.
But before CSS cemented itself, tells us there was RRP, PWP, FOSI, DSSSL, PSL, CHSS, and JSSS.
An awesome journey through web styling history by Zack Bloom.
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The Languages Which Almost Became CSS is a post from CSS-Tricks