Invisible Pink Unicorns - a Firefox emoji rendering bug

Here's a curious bug I just discovered in Firefox 67 for Linux. Can you see this unicorn:
→🦄 ←

What happens if you use CSS to change the opacity of an emoji?

Here's a unicorn, with a pink font colour:

🦄 Unicorn

Let's wrap that in this scrap of CSS to make it 50% opaque.

color: rgba(255, 105, 180, 0.5);

🦄 Unicorn

Hopefully, you see a semi-transparent philosophical argument. What if we set the opacity to 0.0 - that is, completely transparent?

🦄 Unicorn

There's a shunicorn there. If you don't believe me, try highlighting it!

Now, let's try a see-though beastie, but this time adding a drop-shadow:

color: rgba(255, 105, 180, 0.5); text-shadow: .2em .2em .2em #000;

🦄 Unicorn

Good! And now fully transparent text plus a drop-shadow:

color: rgba(255, 105, 180, 0); text-shadow: .2em .2em .2em #000;

🦄 Unicorn

On Firefox, the text is invisible, but the emoji is completely visible - like this...

This bug is not present on Firefox for Android, nor Chromium for Linux.

The only way on Firefox, to get a completely invisible pink unicorn plus a drop shadow is to use:

color: rgba(255, 105, 180, 0.002); text-shadow: .2em .2em .2em #000;

🦄 Unicorn

color: rgba(255, 105, 180, 0.001); text-shadow: .2em .2em .2em #000;

🦄 Unicorn

0.002 is invisible, but 0.001 is visible.

In the meantime, if you can see the invisible pink unicorn, please let me know. Or contact a theologian.

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