CSS two divs next to each other

You can use flexbox to lay out your items:

#parent {
  display: flex;
}
#narrow {
  width: 200px;
  background: lightblue;
  /* Just so it's visible */
}
#wide {
  flex: 1;
  /* Grow to rest of container */
  background: lightgreen;
  /* Just so it's visible */
}

<div id="parent">
  <div id="wide">Wide (rest of width)</div>
  <div id="narrow">Narrow (200px)</div>
</div>

This is basically just scraping the surface of flexbox. Flexbox can do pretty amazing things.

For older browser support, you can use CSS float and a width properties to solve it.

#narrow {
  float: right;
  width: 200px;
  background: lightblue;
}
#wide {
  float: left;
  width: calc(100% - 200px);
  background: lightgreen;
}

<div id="parent">
  <div id="wide">Wide (rest of width)</div>
  <div id="narrow">Narrow (200px)</div>
</div>

I don't know if this is still a current issue or not but I just encountered the same problem and used the CSS display: inline-block; tag. Wrapping these in a div so that they can be positioned appropriately.

<div>
    <div style="display: inline-block;">Content1</div>
    <div style="display: inline-block;">Content2</div>
</div>

Note that the use of the inline style attribute was only used for the succinctness of this example of course these used be moved to an external CSS file.

Unfortunately, this is not a trivial thing to solve for the general case. The easiest thing would be to add a css-style property "float: right;" to your 200px div, however, this would also cause your "main"-div to actually be full width and any text in there would float around the edge of the 200px-div, which often looks weird, depending on the content (pretty much in all cases except if it's a floating image).

EDIT: As suggested by Dom, the wrapping problem could of course be solved with a margin. Silly me.