Four things SVG and CSS did that I did not expect

I spent a while building an icon editor that runs entirely in the browser (icons.jamuny.com, free, no account). Here is what cost me the most time. A presentation attribute loses to any author CSS rule I was scaling handle stroke widths by 1 / zoom and writing the result as an attribute. The value was never used. handle . setAttribute ( ' stroke-width ' , String ( 0.35 / zoom )); .handle { stroke-width: 0.35 } in the stylesheet outranks it, because a presentation attribute sits at the very bottom of the cascade. Measured in Chromium: an attribute of 0.05 computed as 0.35px . Every handle thickened on screen as you zoomed in, for months, with no error anywhere. The fix is a custom property, which is an ordinary declaration and wins where an attribute cannot: layer . style . setProperty ( ' --px ' , String ( 1 / zoom )); /* .handle { stroke-width: calc(0.35 * var(--px)) } */ Geometry attributes like r and width are unaffected. They have no CSS counterpart here, so nothing was ever overriding them. A focused SVG element gets a focus ring measured in user units My canvas is 24 units wide and about 620 pixels. Chrome drew its default focus ring at outline-width: 2.72727px in user units, which is about 24 screen pixels. A fat blue disc appeared around every point you clicked. It was reported to me four times, and four times I thinned something of my own that was not the cause. getComputedStyle ( document . activeElement ). outline That one line found it. My rule only covered :focus-visible , which is the keyboard case, and the keyboard case is the one where I draw a ring of my own. var() does work in a presentation attribute, and I wrote down that it doesn't I needed a segment colour that changes with the theme, so the value is oklch(var(--band-l) var(--band-c) 47) . I applied it through a style and put a comment beside it saying var() is not substituted in presentation attributes. It is. Both forms compute to the same colour, including on an element built detached and ap