Tag:
Web development
Serial:
Not assigned
Publication:
Note
  • Meiqin Yan
  • Pro bono publico

HTML5 is not serializable

Abstract

HTML5 can be serialized in 2 concrete syntaxes. Neither of them can represent all possible HTML5 documents. Suppositions towards a possible general serialization format are provided.

Status of this document

This document provides supplementary documentation regarding standard Web interfaces. It is not an Internet standard, nor is it on the standards track.

Copyright Notice

Copyright (C) The author(s) (2025). All rights reserved.

1. Introduction

HTML5 is the standard language for web documents. It was preceded by XHTML 1.0, and HTML 4.01. It is a superset of neither of these languages; it was designed as a codification of widespread non-standard implementation practice that had diverged significantly from HTML as specified, yet upon which many websites had come to rely.

There are 2 serializations of HTML5 as specified in the standard: the HTML syntax, and the XML syntax. The former syntax is the syntax in which a conformant implementation would parse a document served with the content type "text/html" (no novel content type was specified for HTML5 in the interests of backwards-compatibility); it is the syntax in which the vast majority of webpages are rendered.

The latter syntax was historically referred to as XHTML. Documents in this syntax must be served as "application/xhtml+xml". The vast majority of ostensibly "XHTML" documents -- documents written to be valid XHTML -- are not served as such, and are thus parsed in the HTML5 syntax. Tangentially, this includes the W3C landing webpage located by the XHTML namespace URI itself.

2. Validity versus well-formedness

A document is said to be "valid" HTML5 if it conforms to all the syntactic rules specified in the HTML5 standard. However, the parser for the HTML syntax is required not to reject invalid documents, but to be "forgiving" in that it must be able to generate a document object from any validly-encoded sequence of bytes.

This depreciates the entire notion of "invalidity" in HTML, given that documents with significant structural deformities (such as missing opening and closing <html> tags or <title> elements) are not only expected but required to be parsed into document objects by conformant implementations.

Such documents, which include those that use non-conformant elements such as <center>, are for the purposes of this paper said to be "well-formed", since they can be styled and scripted as if they were valid. In practice, very few web documents are strictly valid HTML.

3. Limitations of the HTML syntax

The fact that the HTML parser is always capable of generating a document tree from a sequence of bytes does not mean that the resulting document tree corresponds to the apparent structure as represented in the markup. The rules for implicit element closing and reopening are complex and not widely understood amongst HTML authors.

It also means that there are some constructs that, while valid in a document abstractly, cannot be represented due to the parser's behavior. For instance, a <p> element is closed upon the encounter of a structural element, such as <blockquote>. While it is possible to have a document in which a <blockquote> is nested within a <p> (and indeed such a document structure can be achieved via scripting), it cannot be serialized in the HTML syntax.

4. XML to the rescue?

Those familiar with XHTML will have already realized that the above problem does not apply to XHTML; the resulting document tree always matches the XML representation exactly, and implicit closing tag shenanigans are not allowed.

Inconveniently, the "forgiving" nature of the HTML parser also leads to the well-formedness of document constructs which are not serializable in XML, chiefly with respect to characters that can occur in tag and attribute names.

Most egregiously, the HTML parser accepts a lesser-than sign character "<" as a character in both element and attribute names. More insidious however is the permissibility of colon characters; this was done so as to maintain code-compatibility with a subset of XHTML, but in doing so broke abstract compatibility of document serializability.

There is no facility for explicit namespacing in the HTML syntax (besides the implicit namespacing of inline SVG and MathML content): every HTML element, known or unknown, is in the same HTML namespace, including those which would have a namespace prefix in XML. Likewise, all element attributes are in no namespace.

While it is technically well-formed in XML for a local name to contain a colon character, it is invalid per the XML namespaces specification. Thus the cruel irony that documents that appear to use XML namespace prefixes which may be perfectly valid XML, but which are parsed in the HTML syntax, result in documents unserializable in the XML syntax.

5. Abandon all hope

Altering the grammatical properties of either extant serialization so as to remedy the problems described herein would be a significant breaking change that would retroactively silently modify the structural properties of a large proportion of the entire web.

On the other hand, it is conceivable that a new syntax could be defined that is capable of representing every HTML5 document.

Existing alternative syntaxes for XML/HTML authoring such as HAML, Emmet expressions or XMQ are primarily oriented towards representing valid HTML5 documents for subsequent compilation into HTML.

The specification for a HTML Semantic Vocabulary currently being undertaken by a W3C community group defines an abstract model based on RDF that can be used to represent the structure of an arbitrary HTML document.

References

  1. WHATWG, "HTML Standard", Living Standard, <http://html.spec.whatwg.org/multipage/>.
  2. W3C, Web Platform WG, "HTML 5.2", 14 December 2017, <http://www.w3.org/TR/2021/SPSD-html52-20210128/>.
  3. W3C, HTML WG, "XHTML™ 1.0: The Extensible HyperText Markup Language", 2nd edition, 1 August 2002, <http://www.w3.org/TR/xhtml1/>.
  4. JTC 1, SC 34, "ISO/IEC 15445:2000 Information technology -- Document description and processing languages -- HyperText Markup Language (HTML)", 2000, <http://www.iso.org/standard/27688.html>.
  5. Catlin, H., Weizenbaum, N., Clarke, N., "Haml (HTML Abstraction Markup Language)", 2006-2023, <http://haml.info/docs/yardoc/file.REFERENCE.html>.
  6. Chikuyonok, S., "Emmet: Tools for web-developers", 2009-2025, <http://emmet.io/>.
  7. Öhrström, F., "XMQ - a configuration language, data-storage language, logging language compatible with xml/html and json", 2025, <http://libxmq.org>.
  8. Bakker, F., "Semantic HTML-vocabulary Community Group", 2022-2025, <http://www.w3.org/community/htmlvoc/>.

Full Copyright Statement

Copyright (C) The author(s) (2025). All Rights Reserved.

Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except in compliance with the License. You may obtain a copy of the License at

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Acknowledgements

HTML and the World Wide Web were invented by Tim Berners-Lee.

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