Div to take up entire remaining width [duplicate]

Answer 1

Do you mean like this?

CSS

html, body {
    height: 100%;
}

#left {
    width:200px;
    float:left;
    background: #f00;
    height: 100%;
}
#right {
    margin-left: 200px;
    background: #0f0;
    height: 100%;
}

Update:

You can also use calc() property in CSS3, which will ease up this process like

html, body {
    height: 100%;
}

#left {
    width:200px;
    float:left;
    background: #f00;
    height: 100%;
}

#right {
    float: left;
    background: #0f0;
    height: 100%;
    width: calc(100% - 200px); /* Negate the fixed width element value from 100% */
}

Demo 2