Aligning a float:left div to center?

use display:inline-block; instead of float

you can't centre floats, but inline-blocks centre as if they were text, so on the outer overall container of your "row" - you would set text-align: center; then for each image/caption container (it's those which would be inline-block;) you can re-align the text to left if you require

CSS Flexbox is well supported these days. Go here for a good tutorial on flexbox.

This works fine in all newer browsers:

#container {
  display:         flex;
  flex-wrap:       wrap;
  justify-content: center;
}

.block {
  width:              150px;
  height:             150px;
  background-color:   #cccccc;
  margin:             10px;        
}

<div id="container">
  <div class="block">1</div>    
  <div class="block">2</div>    
  <div class="block">3</div>    
  <div class="block">4</div>    
  <div class="block">5</div>        
</div>

Some may ask why not use display: inline-block? For simple things it is fine, but if you got complex code within the blocks, the layout may not be correctly centered anymore. Flexbox is more stable than float left.

try it like this:

  <div id="divContainer">
    <div class="divImageHolder">
    IMG HERE
    </div>
    <div class="divImageHolder">
    IMG HERE
    </div>
    <div class="divImageHolder">
    IMG HERE
    </div>
    <br class="clear" />
    </div>

    <style type="text/css">
    #divContainer { margin: 0 auto; width: 800px; }
    .divImageHolder { float:left; }
    .clear { clear:both; }
    </style>