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What is performance loss/gain compared to Javascript solution of this problem?

There's an infinite performance gain for browsers that have javascript turned off or use noscript.

Other than that, I'd say that doing this kind of stuff in CSS would by default be much faster in a web page context.

What if I don't want fixed dimensions? Maybe media queries could help a bit, but Javascript usage seems to be inevitable.

ps. I didn't mean pure javascript solution anyway:)

If you don't care about older browsers [1], you can use Flexbox to do all sorts of dynamic, fluid positioning.

[1] http://caniuse.com/flexbox - it's in the current version of all browsers (some still prefixed).