Think of it this way: use DIVs to group each entity that you want to float:left
.
You want to align the two images, so put each into a div, and float those divs left.
You want to align the box containing the two images with the box containing the video. Float each of those left.
When you use float:left
(or right), the floated DIV is removed from the HTML "flow" and therefore has zero height. Therefore, the DIV containing a floated DIV (or other element) has zero height. Not good. To fix that, apply the clearfix hack to the parent div.
Here is a better explanation (although it uses a different method to clear the floats -- both methods work fine): Customising Insightly HTML contact form (aligned, spaced fields)
/* CSS: */
body{min-width:650px;min-height:650px;}
#pics{float:left;border-radius:5px;XXXmargin-left:50px;border:2px dashed black;}
#picLeft {float:left;height:200px;width:200px;}
#picRight{float:left;height:200px;width:200px;}
#vid{float:left;height:200px;width:200px;border-radius: 5px;border:2px dashed black;}
.clearfix:after{content:"";clear:both;display:block;}
<!-- HTML: -->
<div id="container" class="clearfix">
<div id="pics" class="clearfix">
<div id="picLeft">
<img src="http://placekitten.com/200/200" alt="MyLogo" height="200" width="200" class="mainlogo">
</div>
<div id="picRight">
<img src="http://placekitten.com/195/195" alt="CopeLogo" height="200" width="200" class="exlogo" >
</div>
</div>
<div id="vid">
<iframe src="https://www.youtube.com/watch?v=8E8xMcXmI9E" width="195"></iframe>
</div>
</div>
Additional References:
CSS-Tricks: Clearfix hack
Another example
Ahmed Alaa
's answer also has a good tip: display:inline-block
is also useful - +1