HTML, the Hypertext Markup Language, is a widely used language for structuring content on the World Wide Web. It has evolved over the years, and one aspect of its development is XML compatibility. Here we explore the XML compatibility within the HTML standard as defined by the WHATWG (Web Hypertext Application Technology Working Group) Common Infrastructure and Terminology.
HTML in the XML Namespace
To facilitate the migration from HTML to XML, user agents conforming to the WHATWG specification will place elements in HTML within the http://www.w3.org/1999/xhtml namespace. This mechanism is primarily employed for the Document Object Model (DOM) and Cascading Style Sheets (CSS) purposes. This approach ensures that HTML elements within XML documents are seamlessly integrated with other XML elements. In essence, it harmonizes the treatment of HTML within XML documents.
The HTML namespace is identified by the Uniform Resource Identifier (URI). Elements in this namespace are collectively referred to as "HTML elements." Importantly, this categorization applies to HTML elements, even when they are used within XML documents. This means that any element within the HTML namespace, as long as it is part of an XML document, is considered an "HTML element."
Namespace for Attributes
While the elements themselves are placed within the HTML namespace, attributes within the HTML standard are not associated with a namespace. In other words, attribute names within HTML documents are free from namespace declarations. This simplifies the handling of attributes and helps maintain compatibility with XML.
Element Types in XML Compatibility
In XML compatibility, the term "element type" is employed to define a set of elements that share a common local name and namespace. Essentially, an element type is a group of elements with identical local names and namespaces. For instance, in HTML, "button" elements are considered to belong to the element type "button." This implies that all HTML elements with the local name "button" and within the HTML namespace fall under the element type "button."
XML-Compatible Attribute Names
Attribute names play a vital role in XML compatibility. They must adhere to specific criteria to be considered XML-compatible. According to the WHATWG specification, attribute names are said to be XML-compatible if they meet the following conditions:
- They must match the "Name" production defined in XML. The "Name" production in XML is a formal specification that governs the naming conventions for elements and attributes. It encompasses rules for character usage, such as allowing letters, digits, hyphens, underscores, and periods.
- Attribute names must not contain U+003A COLON characters (:). The colon character is reserved in XML for creating namespaces, and its use within attribute names would introduce conflicts with XML namespaces.
By adhering to these criteria, HTML maintains compatibility with XML while ensuring that attribute names do not interfere with XML namespace conventions.
XML compatibility in the HTML standard, as defined by the WHATWG Common Infrastructure and Terminology, is a crucial aspect of ensuring that HTML and XML can coexist harmoniously. By placing HTML elements in the namespace, categorizing them as "HTML elements" in XML documents, and enforcing XML-compatible attribute names, the specification simplifies the integration of HTML content within XML documents. This approach not only eases migration between the two markup languages but also ensures that the core principles of XML are respected, even in the context of HTML. This compatibility is essential for developers and user agents when working with hybrid HTML and XML content on the web.
