Nothing inherently wrong with an image for a logo, since it works consistently across browsers (both old and new) and looks identical to everyone.
You'll just want the master to be a vector graphic so that you can create multiple scaled raster graphics (e.g. PNGs) to fit different circumstances and usages (since, again, browser support sucks for a lot of vector formats).
CSS Values and Units Level 3 adds the ability to use attr() on any CSS property, not just content, and to use it for non-string values (e.g. numbers, colors).
It's not attr that's unusable, but rather being able to use it on any arbitrary CSS property. Though I'm surprised they don't have a separate entry for just the attr function alone.
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