expr: attribute is an prefix given to HTML Tag Attribute, by giving the attribute this will allowing the HTML Tag used an Blogger Expression.
Blogger Attribute [expr:] Syntax
<div expr:class='data:EXPRESSION' expr:id='data:EXPRESSION'>
<a expr:href='data:EXPRESSION'>
<img expr:src='foo.jpg'/>
</a>
</div>
Blogger Attribute [expr:] Example
Blogger , Tips and Tricks<div expr:class='data:post.id + "article"' expr:id='data:post.id'>
<a expr:href='data:post.url'>
<img expr:src='data:post.thumbnailUrl'/>
</a>
</div>
