Can I use
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Upgrade Insecure Requests

- CR

Declare that browsers should transparently upgrade HTTP resources on a website to HTTPS.

IE

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

Edge

  1. 12 - 16: Not supported
  2. 17 - 85: Supported
  3. 86: Supported

Firefox

  1. 2 - 41: Not supported
  2. 42 - 80: Supported
  3. 81: Supported
  4. 82 - 83: Supported

Chrome

  1. 4 - 42: Not supported
  2. 43 - 85: Supported
  3. 86: Supported
  4. 87 - 89: Supported

Safari

  1. 3.1 - 10: Not supported
  2. 10.1 - 13.1: Supported
  3. 14: Supported
  4. TP: Supported

Opera

  1. 9 - 29: Not supported
  2. 30 - 70: Supported
  3. 71: Supported

iOS Safari

  1. 3.2 - 10.2: Not supported
  2. 10.3 - 13.7: Supported
  3. 14: Supported

Opera Mini

  1. all: Not supported

Android Browser

  1. 2.1 - 4.4.4: Not supported
  2. 81: Supported

Blackberry Browser

  1. 7: Not supported
  2. 10: Not supported

Opera Mobile

  1. 10 - 12.1: Not supported
  2. 59: Supported

Chrome for Android

  1. 85: Supported

Firefox for Android

  1. 79: Supported

IE Mobile

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

UC Browser for Android

  1. 12.12: Not supported

Samsung Internet

  1. 4 - 11.2: Supported
  2. 12.0: Supported

QQ Browser

  1. 10.4: Supported

Baidu Browser

  1. 7.12: Supported

KaiOS Browser

  1. 2.5: Supported

The HTTP header is Content-Security-Policy: upgrade-insecure-requests. Alternatively, the HTML tag is <meta http-equiv="Content-Security-Policy" content="upgrade-insecure-requests">.

Resources:
WebKit feature request bug
MDN Web Docs - Upgrade Insecure Requests
Demo Website