<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>1</title>
</head>
<body>
<div class="percent" style="
width: 250px;
height: 30px;
background: yellow;
border-radius: 10px;
">
</div>
<script>
var percent = document.querySelector(".percent");
percent.style.width =- "50px";
</script>
</body>
</html>
a = a - 1 // тоже самое, что a -= 1
percent.style.width = (parseInt(percent.style.width) - 50) + 'px';