Well, I think it is pretty obvious that both the WebClient and your browser display and parse the web content in a different way because they have been implemented in different ways, by different programmers and different vendors.
But, the question you should be asking yourself is...does it really matter? The semantic and syntactic meaning is exactly the same. So, why do you need to bother about it? Why is it important if it is uppercase or lowercase, or if there's a space after the comma or not? It's not important
By the way, as for the specifications in html, see this quote below taken from the W3C Working Group Note
Tag names for HTML elements may be written with any mix of lowercase and uppercase letters that are a case-insensitive match for the names of the elements given in the HTML elements section of this document; that is, tag names are case-insensitive
Basically, it doesn't matter if it is lowercase or uppercase it is still HTML