It seems as though there's an issue with how Preact handles style props when values are explicitly undefined. It seems to be a little more nuanced than it just leaving undefined in though – I think it happens only when the object property was previously defined, and later becomes undefined:

To Reproduce

• Preact demo
• React demo

Expected behavior

I would expect the --x css property to actually be deleted when it becomes undefined, as this is what happens on initial render.