<input type="password" value="a">

input[type="password"][value$="a"] {
  background-color: blue;
}

https://codepen.io/Jon_codePen/pen/MWYPRjM

https://github.com/maxchehab/CSS-Keylogging

Third party CSS is not safe

https://jakearchibald.com/2018/third-party-css-is-not-safe/