JSFiddle - React, Tailwind, and code Playground

HTML

<div>
<span class="a">something</span>
<span class="b">fill the rest</span> 
</div>

<div>
<span class="a">something</span>
    <span class="b"><input class="c" value="fill the rest" /></span>
</div>

CSS

.a {
    display:table-cell; background-color:red;
}
.b {
    background-color:green;display:table-cell;width:100%;
}

.c {width:100%;}