2047080 - Show warning when attr() is used without type for other properties than `content`

Status: NEW

(DevTools :: Inspector: Rules, enhancement, P2)

Description

[Nicolas Chevobbe [:nchevobbe]]

If you have the following HTML + CSS:

<div data-col=red>Hello</div>

div {
  color: attr(data-col);
}

the color: attr(data-col) declaration doesn't "work", as no type was provided in attr()
The declaration isn't invalid, but it would be nice if we could have a warning somewhere that makes it clear that it won't actually pick the color

(might be something similar to what we need for Bug 2006565)