Markup / CSS / XHTML

Hi,
I am not a web developer, but I am working with the accessibility of document. Someone has commented on the accessibility of our website and wondered if I could get input and feedback from you regarding the comments received. What I am looking for is the validity of the six statements below and what you would suggest for fixes if you are able to. The website was developed by a third party service provider.
Comments received are listed below:

  1.  WCAG 2.0 indicates that websites must have correctly formed markup and CSS, but testing today (2020-02-18) with W3C's markup and CSS validators, I see a total of 447 errors and warnings:

Markup: https://validator.w3.org/nu/?doc=https%3A%2F%2Fwww.durham.ca%2Fen%2Findex.aspx
CSS: https://jigsaw.w3.org/css-validator/validator?uri=https%3A%2F%2Fwww.durham.ca%2Fen%2Findex.aspx
  2.  At the top of Durham.ca's markup, Durham.ca's webmaster has declared "http://www.w3.org/1999/xhtml" as the website's XML namespace, which means that Durham.ca is claiming compliance with W3C's XHTML standard, even though it isn't.  W3C created the XHTML standard in 1999, and finalized it in early 2000, over seventeen years before Durham.ca was revamped in late 2017.  XHTML also predates the existence of Durham.ca, which wasn't created until 2007 as the replacement for Durham.on.ca.
  3.  CSS errors can cause significant usability problems in web browsers and in screen readers that can parse CSS.  If such software/hardware is unable to parse a website's CSS due to errors in the website's CSS, then it will typically respond by not using any of the website's CSS to render its pages.
  4.  Mozilla Firefox is a web browser with a feature that allows a user to temporarily display a website without the website's CSS.  If you use Mozilla Firefox to access Durham.ca, and then go to "View" menu -> "Page Style" -> "No Style", you'll discover that Durham.ca would be extremely difficult to use without CSS.  You can enable CSS again by going to "View" menu -> "Page Style" -> "Basic Page Style".
  5.  While the web browsers and screen readers that Durham.ca's webmaster used to test Durham.ca may have been designed to work around Durham.ca's markup and CSS problems, I'm not aware of any standards that require all web browsers and screen readers to do this, and there's no guarantee that the most popular web browsers and screen readers will continue to tolerate these problems indefinitely.  More importantly, these problems are causing Durham.ca to be non-compliant with AODA.
  6.  Fortunately, all these problems can be corrected fairly easily.  For example, Durham.ca's "meta" markup elements with attributes http-equiv="Content-Style-Type" and http-equiv="Content-Script-Type" are obsolete vestiges from HTML 4 that don't exist in XHTML, so Durham.ca's webmaster could eliminate two markup errors by simply removing these two elements.
Judy Christianson
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