Empty page after installing Angular Playground into an Ionic 4 project

The problem is in a rule coming from Ionic stylesheets you've included as part of the global.scss. If you inspect the body element in browser you'll see it has the following rule attached to it:

html:not(.hydrated) body {
    display: none;
}

This is a rule related to hydration which serves no purpose when using Angular Playground so it can simply be overridden by adding a third stylesheet as src/playground.scss with the following content:

html:not(.hydrated) body {
  display: block;
}

Then add this on top of the ones already in angular.json for the Playground project:

"styles": [
  {
    "input": "src/theme/variables.scss"
  },
  {
    "input": "src/global.scss"
  },
  {
    "input": "src/playground.scss"
  }
],

Now, if you restart the npm run playground it should render the normal Playground page with component selector.

I've also covered this more thoroughly in a blog post.