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input {
  appearance: none;
  background: linear-gradient(to right, #f06 calc(1% * var(--value)), white 0);
}

<input type=range>

for (let input of document.querySelectorAll('input')) {
  input.style.setProperty('--value', input.value);
}

document.addEventListener('input', e => {
  input.style.setProperty('--value', e.target.value);
});