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Comment by Kiro | Hacker News Reader

How come tags are always in uppercase on really old sites and why did the transition to lowercase happen?

According to the standard documents until html4.1 the tags were in uppercase. People followed the standards documents, although there was no explicitly required requirement. Xhtml which came later (2000) explicitly required them to be in lowercase, and now html5 now states that the case is insensitive.

My guess is that back in the day we'd be writing these pages in a plain text editor and all caps made it easier to distinguish tags.

I think that is most of it. Also, upper case was kind of like a virus back then. When you went to write your first page, looked around at what everyone else was doing, it was all caps so you did all caps.

Some people will say it's XHTML. It's not just that. There's always been both in use and HTML has always been case-insensitive.

When Rasmus Lerdorf designed PHP in 1994, he made it have case-insensitive function names since at that time, neither side had won and he wanted it to look good in HTML:

  <CENTER><B>Hello <? ECHO $you >!</B></CENTER>

  <center><b>Hello <? echo $you >!</b></center>

I believe xhtml made uppercase tags invalid.

That is correct. From "XHTML™ 1.0 The Extensible HyperText Markup Language (Second Edition)"

4.2. Element and attribute names must be in lower case

XHTML documents must use lower case for all HTML element and attribute names. This difference is necessary because XML is case-sensitive e.g. <li> and <LI> are different tags.

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