Machine-testable

Content must be accurately marked up according to the relevant standard(s).

Resources

• 0.5% of the Global Top 200 Websites Use Valid HTML
• Every HTML Element
• GPF - General Policy Framework (PDF) - 1.9 - Strategy (Interoperable Technologies)
• GPF - General Policy Framework (PDF) - 4.5 - UX and UI (Native Components)
• GR491 - 1-4006 - Information Over Presentation
• How Often Should We Speak About HTML Conformance?
• HTML: A good basis for accessibility
• HTML5 For Web Designers
• Is React Accessible? That's the Wrong Question
• Jumping HTML tags. Another reason to validate your markup
• Mastering HTML
• Page bloat update
• Plain Vanilla
• Reducing Carbon Emissions On The Web
• Semantic HTML
• Semantic HTML: the foundation of web accessibility
• Ten years of page bloat: What have we learned?
• Testing 10 JavaScript Frameworks on Their HTML Defaults
• The Carbon Impact of Web Standards (PDF)
• Understanding why Semantic HTML is important, as told by TypeScript
• United Nations SDGS - Goal 1 - Poverty
• W3C Validator
• Web Accessibility Lawsuits
• Web Standards: The What, The Why, And The How
• What is page bloat?
• Where Has All the Valid HTML Gone

Machine-testable

Remove optional HTML tags, attribute quotes, and default attributes only when they do not negatively impact functionality, accessibility, or readability. Retain them when they enhance accessibility, maintain clarity (without compromising on performance), or ensure consistent browser rendering.

Resources

• Almost, but not quite, entirely unlike
• Optional HTML: Everything You Need to Know
• Some HTML is “Optional”
• Stop Closing Void Elements
• (Why) Some HTML is "optional"
• You are not required to close your P, LI, IMG, or BR tags in HTML

Machine-testable

Avoid using non-standard elements or attributes.

Resources

• A Node and Command Line Tool to Find Obsolete HTML
• Non-standard elements
• Obsolete and deprecated elements
• Obsolete features
• The Carbon Impact of Web Standards (PDF)
• W3C Validator
• Where Has All the Valid HTML Gone
• Why Do Some HTML Elements Become Deprecated?

Human-testable

Prefer using standard HTML elements and attributes. Only use custom elements or Web Components if you cannot utilize native HTML elements or if you need tightly regulated control over the implementation of design system components..

Resources

• GPF - General Policy Framework (PDF) - 1.9 - Strategy (Interoperable Technologies)
• GPF - General Policy Framework (PDF) - 2.9 - Specifications (Off-The-Shelf Components)
• HTML First
• Plain Vanilla
• Web Components Basics and Performance Benefits
• Web components vs. React
• Why Web Components?

• Environment: Sites with bloated markup waste data, also sites with broken markup could trigger memory leaks (performance issues) in apps, and following standards ensures sites will work the same across devices and platforms (reducing bugs, developer fix time, and resource waste).
• Accessibility: Semantic HTML is a great stepping stone towards making your content easier to navigate by assistive technologies. Many tags within HTML come with pre-loaded context about what is expected within them (reducing the need for ARIA or other descriptive features). This can also help browsers, search engines, social networks, and other "blind" technologies understand your websites or applications better. This can help you reduce barriers in terms of content navigability.
• Performance: Deprecated code isn't optimized within rendering engines, and while Web components do outperform framework components, they won't beat the native HTML elements they build upon.
• Economic: Inaccessible sites can lead to lawsuits, avoiding these is beneficial to any website owner.
• Conversion: Poorly coded sites may break features for visitors, leading to website abandonment.

• code

• content Semantic HTML5 Elements Explained.