User agent stylesheets and border-box

Recently I was trying to troubleshoot why all of the inputs on a project had box-sizing: border-box applied to them. It turns out that a user agent stylesheet rule that I don’t normally see was being applied to all input elements. This user agent stylesheet rule was being applied in Chrome, Firefox, and Safari.

Here is the user agent stylesheet rule that was being applied:

input:not([type="image"]), textarea {
  box-sizing: border-box;
}

After some troubleshooting it turns out this rule was being applied due to a typo in the DOCTYPE for the project. After changing the DOCTYPE back to the correct HTML5 doctype (<!doctype html>) everything loaded correctly.