There is an International reference core standard for HTML Markup: ISO-HTML
The ISO - International Organization for Standardization -- consists of the national standards institutes of 151 countries, on the basis of one member per country, with a Central Secretariat in Geneva, Switzerland. It promulgates, develops and maintains world wide technical standards. The International standard for HTML (International Standard ISO/IEC 15445) was prepared by the Joint Technical Committee ISO/IEC and published in 2000. ISO-HTML is based on W3C HTML 4.01. One of these days - in the future - It will probably be revised and based on HTML5.
I wrote this page that explains the key features of ISO-HTML and provides examples of it's possible use by web developers. It is itself written using validated ISO-HTML Markup and includes links to exemplar pages that use the same content Markup with the document headers changed so that they validate as:
HTML 4.01 (strict)
XHTML 1.0 (strict)
XHTML+RDFa (for those venturing into the Semantic web)
All of these pages are served as content-type text/html with utf-8 character encoding and a CSS3 externally linked style sheet (to include border-radius - I like rounded corners). I have also included a brief java script routine that provides visitor analytics/statistics (StatCounter).
While using ISO-HTML Markup is an excellent way to start off writing standards compliant web pages (with good underpinnings), it's strictness and prohibition against including client-side scripting such as java-script makes it impractical for general use - the W3C recommendations and specifications for the various iterations of HTML should be used instead.
It is important to always validate the pages you produce using the appropriate W3C Validators and correct all errors. Validation enhances Interoperability. However, web pages can, and do, fail validation and still display pretty much as expected in graphical Browsers anyway due to their built-in Markup discrepancy compensation - thereby relying on the often unreliable error correcting properties of individual graphical Browsers. Validation does catch many easily corrected Markup errors and the resultant code is consequently easy to maintain or change. Pages containing invalid Markup may not display or function correctly in Screen Readers, BRAILLE interpreters and Textual Browsers or when incorporated into other applications. As we move toward the Semantic Web, the rigors of XML will result in a greater requirement for Valid Markup. Valid and well formed Markup also bespeaks careful craftsmanship and that appeals to many web authors . IMO it is a very good habit to adopt.
Use the The W3C HTML Markup Validator to check your HTML Markup.