CSS side by side div's auto equal widths

Answer 1

in modern browsers, you can use flexbox

three divs example

two divs example

CSS:

#wrapper {
    display: flex;    
    width:90%;
    height:100px;
    background-color:Gray;
}
#wrapper div {
    flex-basis: 100%;
}

HTML:

    
one one one one one one one one one one one one one one one one one one one one one
    
two two two two two two