Can I use
?

#rrggbbaa hex color notation

- CR

The CSS Color Module Level 4 defines new 4 & 8 character hex notation for color to include the opacity level.

Chrome

  1. ❌ 4 - 51: Not supported
  2. ❌ 52 - 61: Disabled by default
  3. ✅ 62 - 135: Supported
  4. ✅ 136: Supported
  5. ✅ 137 - 139: Supported

Edge

  1. ❌ 12 - 18: Not supported
  2. ✅ 79 - 135: Supported
  3. ✅ 136: Supported

Safari

  1. ❌ 3.1 - 9.1: Not supported
  2. ✅ 10 - 18.4: Supported
  3. ✅ 18.5: Supported
  4. ✅ TP: Supported

Firefox

  1. ❌ 2 - 48: Not supported
  2. ✅ 49 - 137: Supported
  3. ✅ 138: Supported
  4. ✅ 139 - 141: Supported

Opera

  1. ❌ 9 - 38: Not supported
  2. ❌ 39 - 51: Disabled by default
  3. ✅ 52 - 116: Supported
  4. ✅ 117: Supported

IE

  1. ❌ 5.5 - 10: Not supported
  2. ❌ 11: Not supported

Chrome for Android

  1. ✅ 136: Supported

Safari on iOS

  1. ❌ 3.2 - 9.3: Not supported
  2. ✅ 10 - 18.4: Supported
  3. ✅ 18.5: Supported

Samsung Internet

  1. ❌ 4: Not supported
  2. ❌ 5 - 7.4: Disabled by default
  3. ✅ 8.2 - 26: Supported
  4. ✅ 27: Supported

Opera Mini

  1. ❌ all: Not supported

Opera Mobile

  1. ❌ 10 - 12.1: Not supported
  2. ✅ 80: Supported

UC Browser for Android

  1. ✅ 15.5: Supported

Android Browser

  1. ❌ 2.1 - 4.4.4: Not supported
  2. ✅ 136: Supported

Firefox for Android

  1. ✅ 137: Supported

QQ Browser

  1. ✅ 14.9: Supported

Baidu Browser

  1. ✅ 13.52: Supported

KaiOS Browser

  1. ❌ 2.5: Not supported
  2. ✅ 3: Supported

Only supported in Android WebViews in apps that target Android Pie, due to this issue

Resources:
JS Bin testcase