You can do this using pure CSS; no JavaScript needed. This utilizes the (somewhat counterintuitive) fact that padding-top
percentages are relative to the containing block’s width. Here’s an example:
.wrapper {
width: 50%;
/* whatever width you want */
display: inline-block;
position: relative;
}
.wrapper:after {
padding-top: 56.25%;
/* 16:9 ratio */
display: block;
content: '';
}
.main {
position: absolute;
top: 0;
bottom: 0;
right: 0;
left: 0;
/* fill parent */
background-color: deepskyblue;
/* let's see it! */
color: white;
}
<div class="wrapper">
<div class="main">
This is your div with the specified aspect ratio.
</div>
</div>