[css-properties-values-api] JS API should use CSSOMString #1099
Description
https://drafts.css-houdini.org/css-properties-values-api-1/#registering-custom-properties should probably use CSSOMString, since it's a CSSOM API, effectively.
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https://drafts.css-houdini.org/css-properties-values-api-1/#registering-custom-properties should probably use CSSOMString, since it's a CSSOM API, effectively.