// source https://jsbin.com/muyedif

index.html

<style id="jsbin-css">
.container {
    width: 100%;
    border: 1px solid #cec;
    margin: 10px;
}

.container .img {
    width: 100%;
}
</style>
<div class="container">
    <img class="img" src="https://dummyimage.com/100x100/ededed/c1c2c9.png">
</div>
<script id="jsbin-source-html" type="text/html"><div class="container">
    <img class="img" src="https://dummyimage.com/100x100/ededed/c1c2c9.png">
</div>
</script>

<script id="jsbin-source-css" type="text/css">.container {
    width: 100%;
    border: 1px solid #cec;
    margin: 10px;
}

.container .img {
    width: 100%;
}</script>

jsbin.muyedif.css

.container {
    width: 100%;
    border: 1px solid #cec;
    margin: 10px;
}

.container .img {
    width: 100%;
}