Html Details - What Are They?

[GSD4ME]

Something that has got me stumped in HTML - not the actual display or anything but the syntactic and semantic meaning of something.
I needed to analyse some HTML - which I understand and have various books on the subject - but none of them list these 'extras' that I am seeing or explain what they are.Internet searches haven't come up with anything either (yet!)
In the HTML I am looking at, nearly every close tag is followed by an 'at' sign and a list of numbers, separated by a dot character - for example @0.1.4.1.2.0.0.0.0.0.
Can someone please explain what they are, what they mean, what interpretation they imply and anything else applicable to them. Many thanks