The HTML specification introduces a comprehensive set of conformance requirements for authors. These requirements are vital to ensure that web documents are created in a consistent and standardized manner. While previous versions of HTML focused primarily on valid documents, now addresses both valid and invalid documents, emphasizing the importance of conformance.
Presentational Markup
One of the significant changes introduced in current HTML is the reduction of presentational markup. Presentational markup, which includes elements used for styling and formatting, has been discouraged due to several reasons:
- Accessibility: Presentational elements can lead to poorer accessibility for users, especially those relying on assistive technologies. Using semantically-appropriate markup enhances accessibility for all users.
- Maintenance: Maintaining websites with style-independent markup is more straightforward. Changes in presentation, such as color modifications, are easily accomplished with CSS styles rather than modifying individual presentational elements.
- Document Size: Presentational markup tends to be redundant, resulting in larger document sizes. Using media-independent markup reduces document bloat.
Current HTML retains only a few presentational features, such as the `style` attribute and the `style` element. The `style` attribute, while discouraged in production environments, can be useful for rapid prototyping or specific styles in rare cases. The `style` element is suitable for syndication or page-specific styles, but external style sheets are generally more convenient for multiple pages.
Some elements that were previously presentational have been redefined to be media-independent, including `b`, `i`, `hr`, `s`, `small`, and `u`.
Syntax Errors
Current HTML places constraints on the HTML syntax to prevent various issues:
- Unintuitive Error Handling: Certain invalid syntax constructs can result in unintuitive Document Object Model (DOM) trees. Ensuring intuitive parsing is crucial for consistent web rendering.
- Optional Error Recovery: User agents are allowed to fail when encountering parse errors to maintain compatibility with controlled environments where complex error handling is unnecessary.
- Incompatible Behavior: Some error-handling behaviors are incompatible with streaming user agents. These behaviors are considered invalid to ensure interoperability.
- Infoset Coercion: Syntax constructs that violate XML invariants when connected to an HTML parser may require the parser to coerce the HTML DOM into an XML-compatible infoset, making them invalid.
- Poor Performance: Certain syntax constructs result in poor performance, and to discourage their use, they are made non-conforming.
- Fragile Syntax Constructs: Fragile syntax constructs, such as named character references without semicolons, are made non-conforming to avoid unintended interpretations.
- Interoperability Problems: Certain syntax constructs known to cause problems in legacy user agents are marked as non-conforming to prevent subtle issues.
- Security Concerns: Restrictions, such as the prohibition of UTF-7 usage, are in place to prevent known security vulnerabilities.
- Author's Intent Unclear: Markup with unclear author intent is made non-conforming to enhance clarity and ease of maintenance.
- Likely Typos: Simple typos, such as misspelled element or attribute names, are considered errors to save authors debugging time.
- Future Syntax Compatibility: Certain otherwise harmless features are disallowed to allow for future language syntax extensions.
Restrictions on Content Models and Attribute Values
Current HTML imposes restrictions on how elements and attributes can be specified for the following reasons:
- Dubious Semantics: Content models restrict nesting when it might lead to dubious semantic use of elements, preventing misuse.
- Conflicts in Semantics: Clear contradictions in semantics expressed are considered conformance errors.
- Confusing Default Styles: Elements with default styles or behaviors that lead to confusion are disallowed when alternatives exist.
- Author Misunderstanding: Disallowing elements or attributes that could cause author confusion is vital.
- Simplified Language Learning: Some conformance errors simplify the language for authors.
- Parser Peculiarities: Elements parsed eccentrically have content model restrictions to prevent exposure to parsing issues.
- Script Failure Prevention: Some errors help prevent script issues that are challenging to debug.
- Time-Saving: Certain constructs are disallowed to save authoring time and avoid common pitfalls.
- Transitionary Use: Some restrictions facilitate authors using files interpretable as both XML and HTML.
- Future Expansion: Restrictions on content models and attribute values allow for future expansion of HTML vocabulary.
- Support for Other Specifications: Restrictions support conformance rules from other specifications.
