I think it’s much better if you use classes for this purpose.
Please, try this solution:
<style>
.img {
max-width: 100%;
max-height: 100%;
width: auto;
height: auto;
}
.changesize {
max-width: 40%;
max-height: 40%;
}
</style>
<div id="win" class="img">xyz</div>
<script>
document.getElementById("win").className = "changesize";
</script>
Hope it helps.
solved Change DIV Image Style with Javascript