::-webkit-meter-suboptimum-value

::-webkit-meter-suboptimum-value {
  /* ... */
}

Normal:
<meter min="0" max="10" low="3" high="7" optimum="6" value="2">
  Score 2/10
</meter>
<br />
Styled:
<meter id="styled" min="0" max="10" low="3" high="7" optimum="6" value="2">
  Score 2/10
</meter>

body {
  font-family: monospace;
}
.safari meter {
  /* Reset the default appearance for Safari only */
  /* .safari class is added via JavaScript */
  -webkit-appearance: none;
}
#styled::-webkit-meter-suboptimum-value {
  background: linear-gradient(
    to bottom,
    #ffff77,
    #999900 45%,
    #999900 55%,
    #ffff77
  );
  height: 100%;
  box-sizing: border-box;
}

// Safari requires <meter> elements to have an `appearance` of `none` for custom styling
// using `::-webkit-meter-*` selectors, but `appearance: none` breaks rendering on Chrome.
// Therefore, we must check if the browser is Safari-based.
const is_safari =
  navigator.userAgent.includes("AppleWebKit/") &&
  !navigator.userAgent.includes("Chrome/");
if (is_safari) {
  document.body.classList.add("safari");
}