Fixed Element to Container on Page

by Ian Roberts

HTML

<div class="fixedThing"> I am a fixed thing</div>
<div style="width:500px;background-color:#bbb;margin:0px auto;">I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/>I am a site container...<br/></div>

CSS

.fixedThing {
  background-color:#ddd;
  position: fixed;
  width: 100px;
  height: 200px;
  left: 50%;
  top: 0%;
  margin-left: 260px; /*half width of siteContainer + 10px to be smart*/
}