Given it doesn't work at all I'd rather keep this as false and enable on the relevant tests.

Probably:

let omit_color_interpolation_method = matches!(color_interpolation_method.space, ColorSpace::Srgb);
if !omit_color_interpolation_method {
    if !omit_shape || !omit_position {
        ...
    ...
}

FnOnce is probably more reflective of what you want to do, though it doesn't matter here in practice.

I think this would be a bit cleaner with something like:

fn try_parse_interpolation_method(&ParserContext, &mut Parser..) -> Option<ColorInterpolationMethod> {
    if !gradient_color_interpolation_method_enabled() {
        return None;
    }
    input.try_parse(|p| ColorInterpolationMethod::parse(context, p)).ok()
}

Then you probably don't need this function at all, or it's not so useful. You can just:

let mut color_interpolation_method = Self::try_parse_interpolation_method(...);
let direction = input.try_parse(|i| LineDirection::parse(context, i, &mut compat_mode)).ok();
if color_interpolation_method.is_none() && direction.is_some() {
    color_interpolation_method = Self::try_parse_interpolation_method(..);
}

For example. But it's fine to keep this extra function if you want too.

Why are these failing?

Curious why do these not work?

Same, shouldn't these work? If not, why not/

Right now we omit the color space if it is srgb, even if it was specified like this.

The correct method involves calculating the default color space which could be oklab() or srgb(). Then when the specified color space matches the default, it should be omitted, but we don't do that yet.

Same as above. In this case, oklab() is the default color space and should be omitted, but we only use srgb as the default, so the color space is present in serialization.

Same again, because oklab() is the default here, so should be omitted, but it's not.