CSS vars set in the style attribute via JS appears to not be preserved.

[Ampflower]

Relevant section of code:

https://github.com/Ampflower/ampflower.gay/blob/951fbd1e764a14e767070c9903ea7e017d1a3de4/src/assets/js/hibiscus.js#L63-L66

It is notable that inspect element can override it and preserve it perfectly fine, telling me this is likely a JS API problem and not a CSS or DOM problem.

[AtkinsSJ]

I think this is the relevant code:

https://github.com/LadybirdBrowser/ladybird/blob/e40ad73ae79023f64e250f854d0730c21e0f54fc/Userland/Libraries/LibWeb/CSS/CSSStyleDeclaration.cpp#L291-L297

CSS variables (technical term "custom properties") will return nothing from property_id_from_string() so it's not doing anything.

A rough idea of what needs changing:

• Checks there for a custom property name, and probably pass an Optional<FlyString> custom_property_name to the other set_property() method.
• PropertyOwningCSSStyleDeclaration::set_a_css_declaration() also needs to take the custom property name, and store it in m_custom_properties instead of m_properties.