JSFiddle - React, Tailwind, and code Playground

HTML

<body>
    <div id='wrapper'>
         <div id='a'>Some content that is long enough to fill a full line an even more, so you need to go back to the next line, … Ok, I think that is enough. If you have a wider screen, append text there :)
        </div>
        <div id='b'> I want this div to fill all the empty save, not more.
        </div>
    </div>
</body>

CSS

#wrapper {
    position: absolute;
    top:0;
    height: 100%;
    width: 70%;
    display: flex;
    flex-flow: column;
}

#a {
    background-color: grey;
    flex: 0 1 auto;
}

#b {
    background-color: lightblue;
    flex: 1 1 auto;
}