Consider adding an `:installed` pseudoclass for styling

[tomayac]

When an app is already installed, the install button rarely makes sense to display and you'd typically just hide it. A nice declarative way of doing so could be via a CSS pseudoclass:

install:installed {
  display: none;
}

[mikewest]

This seems reasonable to want. At the same time, it seems reasonable for users to be reluctant to reveal the set of PWAs they've installed. Currently, the goal is to avoid that even through implicit channels (e.g. the element width if we shift from "Install" to "Launch" for installed apps).

Do you have thoughts about the information leak here that would cause us to reevaluate in favor of developer interest?

[tomayac]

I was mostly just thinking about the current site use case. :installed could be defined as being only exposed on the current site.

[robpaveza]

While I'm sure we could block out the relevant deltas within getComputedStyle, another option that we discussed in this direction would be to have some kind of behavior based on an attribute. For example:

<install></install>
<install displaymode="show-installed"></install>
<install displaymode="hide-when-installed"></install>
<install displaymode="open-in-browser"></install>

I think we'd need to work through the specifics here, but at a high level, I think we'd only change the appearance of the element when it's running inside of an installed and open app. Within a browser tab, I think it would always just show "Install" or "Installed" and be able to launch the app.