Can I use
?

CSS3 Opacity

- REC

Method of setting the transparency level of an element

Chrome

  1. ✅ 4 - 142: Supported
  2. ✅ 143: Supported
  3. ✅ 144 - 146: Supported

Edge

  1. ✅ 12 - 142: Supported
  2. ✅ 143: Supported

Safari

  1. ✅ 3.1 - 26.1: Supported
  2. ✅ 26.2: Supported
  3. ✅ TP: Supported

Firefox

  1. ✅ 2 - 145: Supported
  2. ✅ 146: Supported
  3. ✅ 147 - 149: Supported

Opera

  1. ✅ 9 - 124: Supported
  2. ✅ 125: Supported

IE

  1. ◐ 5.5 - 8: Partial support
  2. ✅ 9 - 10: Supported
  3. ✅ 11: Supported

Chrome for Android

  1. ✅ 143: Supported

Safari on iOS

  1. ✅ 3.2 - 18.7: Supported
  2. ✅ 26.0: Supported
  3. ✅ 26.1: Supported

Samsung Internet

  1. ✅ 4 - 28: Supported
  2. ✅ 29: Supported

Opera Mini

  1. ✅ all: Supported

Opera Mobile

  1. ✅ 10 - 12.1: Supported
  2. ✅ 80: Supported

UC Browser for Android

  1. ✅ 15.5: Supported

Android Browser

  1. ✅ 2.1 - 4.4.4: Supported
  2. ✅ 143: Supported

Firefox for Android

  1. ✅ 146: Supported

QQ Browser

  1. ✅ 14.9: Supported

Baidu Browser

  1. ✅ 13.52: Supported

KaiOS Browser

  1. ✅ 2.5: Supported
  2. ✅ 3: Supported

Transparency for elements in IE8 and older can be achieved using the proprietary "filter" property and does not work well with PNG images using alpha transparency.

Resources:
MDN - opacity
WebPlatform Docs