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 - 116: Supported
  4. 117: Supported
  5. 118 - 120: Supported

Edge

  1. 12 - 18: Not supported
  2. 79 - 116: Supported
  3. 117: Supported

Safari

  1. 3.1 - 9.1: Not supported
  2. 10 - 16.6: Supported
  3. 17.0: Supported
  4. 17.1 - TP: Supported

Firefox

  1. 2 - 48: Not supported
  2. 49 - 116: Supported
  3. 117: Supported
  4. 118 - 120: Supported

Opera

  1. 9 - 38: Not supported
  2. 39 - 51: Disabled by default
  3. 52 - 101: Supported
  4. 102: Supported

IE

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

Chrome for Android

  1. 117: Supported

Safari on iOS

  1. 3.2 - 9.3: Not supported
  2. 10 - 16.6: Supported
  3. 17.0: Supported
  4. 17.1: Supported

Samsung Internet

  1. 4: Not supported
  2. 5 - 7.4: Disabled by default
  3. 8.2 - 21: Supported
  4. 22: Supported

Opera Mini

  1. all: Not supported

Opera Mobile

  1. 10 - 12.1: Not supported
  2. 73: Supported

UC Browser for Android

  1. 15.5: Supported

Android Browser

  1. 2.1 - 4.4.4: Not supported
  2. 117: Supported

Firefox for Android

  1. 117: Supported

QQ Browser

  1. 13.1: Supported

Baidu Browser

  1. 13.18: 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