<!DOCTYPE html>
<html>
    <body>
        <div id="outer" style="height: 50vh; width: 50vw; -webkit-box-sizing:border-box; background-color: lightcyan; border: solid 1px black;" >
            <div id="inner" style="width: 50%; height: 50%; background-color: lightseagreen; border: solid 1px black;"></div>
        </div>
    </body>
</html>