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 

<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>

Other Attribute

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