attr.maybe should offer similar API to attr

[raquo]

Currently only attr.maybe(someValue) is possible, but not attr.maybe := someValue or attr.maybe <-- $someValue. We could implement these with some work.

The main problem is, we don't know the default value of each attr / prop / css key.

But...

• For keys that are Attr or ReflectedAttr, we could probably use Javascript's removeAttribute method
• For CSS props we could probably use CSSStyleDeclaration.removeProperty
• This leaves pure, non-attribute props, but we only have a handful of them, so although there is no generic solution, false and "" will work for those specific props.

Main complication is that ReflectedAttrs currently uses the Props class from SDT which doesn't carry any info about the corresponding attribute name. But SDT itself does provide this information. So we will probably need a custom Props class in Laminar for this.

We could also potentially use the property's current value (at the time the maybe modifier is added) as its default value, but I could see this being confusing if the user has previously set some value to this property. But actually, if we don't do this, the behaviour of attr.maybe(someValue) and attr.maybe <-- Val(someValue) would be different. Hm...