XHTML validator

Hello 
I'm an instructor and make my students validate using your website as they
are coding by hand.   I've had 2 students one last week and one this week -
when they validated - once had FAILURE with errors/warnings - then later on
- did it again - and it was perfect. 
Can you tell me if you're updating or changing anything that I need to be
aware of ?    I have 20 students and they usually have 3-4 assignments with
multiple pages they check before turning in and I recheck for grading.
I love your site and really drill into their heads - if you are gonna do it
- do it right the first time!!  Thanks for your assistance with XHTML and
CSS Validation.
 
Lisa Troxler
636-447-3347
Instructor at Webster University and St Charles Community College