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Parsing an HTML5 Document

Nokogiri provides the ability for a Ruby program to invoke our version of the Gumbo HTML5 parser and to access the result as a Nokogiri::HTML::Document.

Why is HTML5 parsing covered as a separate tutorial from HTML parsing?

Eventually, we'll integrate HTML5 docs into the other of tutorials, and this page will go away. We shipped HTML5 support as quickly as possible by copying (and only lightly editing) the Nokogumbo documentation to this page.

HTML5 support is only in v1.12.0 and later.

This tutorial describes functionality that is only available in Nokogiri v1.12.0 and later. Please visit the HTML5 API documentation for more information.

HTML5 functionality is not available when running JRuby.

The JRuby implementation of Nokogiri does not support HTML5 as of v1.12.0. If you'd like to help implement this support, or would like to know more, please see #2227.


Parse an HTML5 document:

doc = Nokogiri.HTML5(string)

Parse an HTML5 fragment:

fragment = Nokogiri::HTML5.fragment(string)

Parsing options

The document and fragment parsing methods support options that are different from Nokogiri's.

  • Nokogiri.HTML5(html, url = nil, encoding = nil, options = {})
  • Nokogiri::HTML5.parse(html, url = nil, encoding = nil, options = {})
  • Nokogiri::HTML5::Document.parse(html, url = nil, encoding = nil, options = {})
  • Nokogiri::HTML5.fragment(html, encoding = nil, options = {})
  • Nokogiri::HTML5::DocumentFragment.parse(html, encoding = nil, options = {})

The three currently supported options are :max_errors, :max_tree_depth and :max_attributes, described below.

Error reporting

Nokogiri contains an experimental HTML5 parse error reporting facility. By default, no parse errors are reported but this can be configured by passing the :max_errors option to HTML5.parse or HTML5.fragment.

For example, this script:

doc = Nokogiri::HTML5.parse('<span/>Hi there!</span foo=bar />', max_errors: 10)
doc.errors.each do |err|


1:1: ERROR: Expected a doctype token
<span/>Hi there!</span foo=bar />
1:1: ERROR: Start tag of nonvoid HTML element ends with '/>', use '>'.
<span/>Hi there!</span foo=bar />
1:17: ERROR: End tag ends with '/>', use '>'.
<span/>Hi there!</span foo=bar />
1:17: ERROR: End tag contains attributes.
<span/>Hi there!</span foo=bar />

Using max_errors: -1 results in an unlimited number of errors being returned.

The HTML standard defines a number of standard parse error codes. These error codes only cover the "tokenization" stage of parsing HTML. The parse errors in the "tree construction" stage do not have standardized error codes (yet).

As a convenience to Nokogiri users, the defined error codes are available via Nokogiri::XML::SyntaxError#str1 method:

doc = Nokogiri::HTML5.parse('<span/>Hi there!</span foo=bar />', max_errors: 10)
doc.errors.each do |err|
  puts("#{err.line}:#{err.column}: #{err.str1}")
# => 1:1: generic-parser
#    1:1: non-void-html-element-start-tag-with-trailing-solidus
#    1:17: end-tag-with-trailing-solidus
#    1:17: end-tag-with-attributes

Note that the first error is generic-parser because it's an error from the tree construction stage and doesn't have a standardized error code.

For the purposes of semantic versioning, the error messages, error locations, and error codes are not part of Nokogiri's public API. That is, these are subject to change without Nokogiri's major version number changing. These may be stabilized in the future.

Maximum tree depth

The maximum depth of the DOM tree parsed by the various parsing methods is configurable by the :max_tree_depth option. If the depth of the tree would exceed this limit, then an ::ArgumentError is thrown.

This limit (which defaults to Nokogiri::Gumbo::DEFAULT_MAX_TREE_DEPTH = 400) can be removed by giving the option max_tree_depth: -1.

html = '<!DOCTYPE html>' + '<div>' * 1000
doc = Nokogiri.HTML5(html)
# raises ArgumentError: Document tree depth limit exceeded
doc = Nokogiri.HTML5(html, max_tree_depth: -1)

Attribute limit per element

The maximum number of attributes per DOM element is configurable by the :max_attributes option. If a given element would exceed this limit, then an ::ArgumentError is thrown.

This limit (which defaults to Nokogiri::Gumbo::DEFAULT_MAX_ATTRIBUTES = 400) can be removed by giving the option max_attributes: -1.

html = '<!DOCTYPE html><div ' + (1..1000).map { |x| "attr-#{x}" }.join(' ') + '>'
# "<!DOCTYPE html><div attr-1 attr-2 attr-3 ... attr-1000>"
doc = Nokogiri.HTML5(html)
# raises ArgumentError: Attributes per element limit exceeded
doc = Nokogiri.HTML5(html, max_attributes: -1)

HTML Serialization

After parsing HTML, it may be serialized using any of the Nokogiri::XML::Node serialization methods. In particular, XML::Node#serialize, #to_html, and #to_s will serialize a given node and its children. (This is the equivalent of JavaScript's Element.outerHTML.) Similarly, XML::Node#inner_html will serialize the children of a given node. (This is the equivalent of JavaScript's Element.innerHTML.)

doc = Nokogiri::HTML5("<!DOCTYPE html><span>Hello world!</span>")
puts doc.serialize
# => <!DOCTYPE html><html><head></head><body><span>Hello world!</span></body></html>


  • The Nokogiri::HTML5.fragment function takes a string and parses it as a HTML5 document. The <html>, <head>, and <body> elements are removed from this document, and any children of these elements that remain are returned as a Nokogiri::HTML5::DocumentFragment.

  • The Nokogiri::HTML5.parse function takes a string and passes it to the gumbo_parse_with_options method, using the default options. The resulting Gumbo parse tree is then walked.

  • Instead of uppercase element names, lowercase element names are produced.

  • Instead of returning unknown as the element name for unknown tags, the original tag name is returned verbatim.