diff --git a/Gemfile b/Gemfile
index d65e2a66..fa75df15 100644
--- a/Gemfile
+++ b/Gemfile
@@ -1,3 +1,3 @@
-source 'http://rubygems.org'
+source 'https://rubygems.org'
 
 gemspec
diff --git a/README.md b/README.md
index 8b5e4b80..932984bc 100644
--- a/README.md
+++ b/README.md
@@ -61,7 +61,7 @@ You can enable or disable most of the following checks.
 
 ### HTML
 
-* Whether your HTML markup is valid. This is done via [Nokogiri](http://www.nokogiri.org/tutorials/ensuring_well_formed_markup.html), to ensure well-formed markup.
+* Whether your HTML markup is valid. This is done via [Nokogumbo](https://github.com/rubys/nokogumbo) to validate well-formed HTML5 markup.
 
 ## Usage
 
@@ -196,7 +196,7 @@ htmlproofer ./_site
 
 ### Using through Docker
 
-If you have trouble with (or don't want to) install Ruby/Nokogiri, the command-line tool can be run through Docker. See [html-proofer-docker](https://github.com/18F/html-proofer-docker) for more information.
+If you have trouble with (or don't want to) install Ruby/Nokogumbo, the command-line tool can be run through Docker. See [html-proofer-docker](https://github.com/18F/html-proofer-docker) for more information.
 
 ## Ignoring content
 
@@ -218,7 +218,7 @@ The `HTMLProofer` constructor takes an optional hash of additional options:
 | `check_external_hash` | Checks whether external hashes exist (even if the webpage exists). This slows the checker down. | `false` |
 | `check_favicon` | Enables the favicon checker. | `false` |
 | `check_opengraph` | Enables the Open Graph checker. | `false` |
-| `check_html` | Enables HTML validation errors from Nokogiri | `false` |
+| `check_html` | Enables HTML validation errors from Nokogumbo | `false` |
 |`checks_to_ignore`| An array of Strings indicating which checks you'd like to not perform. | `[]`
 | `directory_index_file` | Sets the file to look for when a link refers to a directory. | `index.html` |
 | `disable_external` | If `true`, does not run the external link checker, which can take a lot of time. | `false` |
@@ -247,13 +247,13 @@ See below for more information.
 
 ### Configuring HTML validation rules
 
-If `check_html` is `true`, Nokogiri performs additional validation on your HTML.
+If `check_html` is `true`, Nokogumbo performs additional validation on your HTML.
 
 You can pass in additional options to configure this validation.
 
 | Option | Description | Default |
 | :----- | :---------- | :------ |
-| `report_invalid_tags` | When `check_html` is enabled, HTML markup that is unknown to Nokogiri are reported as errors. | `false`
+| `report_invalid_tags` | When `check_html` is enabled, HTML markup that is unknown to Nokogumbo are reported as errors. | `false`
 | `report_missing_names` | When `check_html` is enabled, HTML markup that are missing entity names are reported as errors. | `false`
 | `report_script_embeds` | When `check_html` is enabled, `script` tags containing markup [are reported as errors](http://git.io/vOovv). Enabling this option ignores those errors. | `false`
 
diff --git a/bin/htmlproofer b/bin/htmlproofer
index ba67e59f..73d6d5c0 100755
--- a/bin/htmlproofer
+++ b/bin/htmlproofer
@@ -20,7 +20,7 @@ Mercenary.program(:htmlproofer) do |p|
   p.option 'checks_to_ignore', '--checks-to-ignore check1,[check2,...]', Array, ' An array of Strings indicating which checks you\'d like to not perform.'
   p.option 'check_external_hash', '--check-external-hash', 'Checks whether external hashes exist (even if the webpage exists). This slows the checker down (default: `false`).'
   p.option 'check_favicon', '--check-favicon', 'Enables the favicon checker (default: `false`).'
-  p.option 'check_html', '--check-html', 'Enables HTML validation errors from Nokogiri (default: `false`).'
+  p.option 'check_html', '--check-html', 'Enables HTML validation errors from Nokogumbo (default: `false`).'
   p.option 'check_img_http', '--check-img-http', 'Fails an image if it\'s marked as `http` (default: `false`).'
   p.option 'check_opengraph', '--check-opengraph', 'Enables the Open Graph checker (default: `false`).'
   p.option 'directory_index_file', '--directory-index-file', String, 'Sets the file to look for when a link refers to a directory. (default: `index.html`)'
@@ -32,9 +32,10 @@ Mercenary.program(:htmlproofer) do |p|
   p.option 'external_only', '--external_only', 'Only checks problems with external references'
   p.option 'file_ignore', '--file-ignore file1,[file2,...]', Array, 'A comma-separated list of Strings or RegExps containing file paths that are safe to ignore'
   p.option 'http_status_ignore', '--http-status-ignore 123,[xxx, ...]', Array, 'A comma-separated list of numbers representing status codes to ignore.'
-  p.option 'report_invalid_tags', '--report-invalid-tags', 'Ignore `check_html` errors associated with unknown markup (default: `false`)'
-  p.option 'report_missing_names', '--report-missing-names', 'Ignore `check_html` errors associated with missing entities (default: `false`)'
-  p.option 'report_script_embeds', '--report-script-embeds', 'Ignore `check_html` errors associated with `script`s (default: `false`)'
+  p.option 'report_invalid_tags', '--report-invalid-tags', 'Report `check_html` errors associated with unknown markup (default: `false`)'
+  p.option 'report_missing_names', '--report-missing-names', 'Report `check_html` errors associated with missing entities (default: `false`)'
+  p.option 'report_script_embeds', '--report-script-embeds', 'Report `check_html` errors associated with `script`s (default: `false`)'
+  p.option 'report_missing_doctype', '--report-missing-doctype', 'Report `check_html` errors associated with missing or out-of-order DOCTYPE (default: `false`)'
   p.option 'log_level', '--log-level <level>', String, 'Sets the logging level, as determined by Yell'
   p.option 'only_4xx', '--only-4xx', 'Only reports errors for links that fall within the 4xx status code range'
   p.option 'timeframe', '--timeframe <time>', String, 'A string representing the caching timeframe.'
@@ -76,6 +77,7 @@ Mercenary.program(:htmlproofer) do |p|
     options[:validation][:report_script_embeds] = opts['report_script_embeds']
     options[:validation][:report_missing_names] = opts['report_missing_names']
     options[:validation][:report_invalid_tags] = opts['report_invalid_tags']
+    options[:validation][:report_missing_doctype] = opts['report_missing_doctype']
 
     options[:cache] = {}
     options[:cache][:timeframe] = opts['timeframe'] unless opts['timeframe'].nil?
diff --git a/html-proofer.gemspec b/html-proofer.gemspec
index d2182be6..8e3c4c89 100644
--- a/html-proofer.gemspec
+++ b/html-proofer.gemspec
@@ -18,7 +18,7 @@ Gem::Specification.new do |gem|
   gem.require_paths = ['lib']
 
   gem.add_dependency 'mercenary',       '~> 0.3.2'
-  gem.add_dependency 'nokogiri',        '~> 1.5'
+  gem.add_dependency 'nokogumbo',       '~> 1.4', '>= 1.4.10'
   gem.add_dependency 'colored',         '~> 1.2'
   gem.add_dependency 'typhoeus',        '~> 0.7'
   gem.add_dependency 'yell',            '~> 2.0'
diff --git a/lib/html-proofer/check/html.rb b/lib/html-proofer/check/html.rb
index 38c366aa..0d99fc15 100644
--- a/lib/html-proofer/check/html.rb
+++ b/lib/html-proofer/check/html.rb
@@ -1,26 +1,29 @@
 class HtmlCheck < ::HTMLProofer::Check
+  # tags embedded in scripts are used in templating languages: http://git.io/vOovv
   SCRIPT_EMBEDS_MSG = /Element script embeds close tag/
   INVALID_TAG_MSG = /Tag ([\w\-:]+) invalid/
   INVALID_PREFIX = /Namespace prefix/
   PARSE_ENTITY_REF = /htmlParseEntityRef: no name/
+  DOCTYPE_MSG = /The doctype must be the first token in the document/
 
   def run
     @html.errors.each do |error|
-      message = error.message
-      line    = error.line
-
-      if message =~ INVALID_TAG_MSG || message =~ INVALID_PREFIX
-        next unless options[:validation][:report_invalid_tags]
-      end
-
-      if message =~ PARSE_ENTITY_REF
-        next unless options[:validation][:report_missing_names]
-      end
-
-      # tags embedded in scripts are used in templating languages: http://git.io/vOovv
-      next if !options[:validation][:report_script_embeds] && message =~ SCRIPT_EMBEDS_MSG
+      add_issue(error.message, line: error.line) if report?(error.message)
+    end
+  end
 
-      add_issue(message, line: line)
+  def report?(message)
+    case message
+    when SCRIPT_EMBEDS_MSG
+      options[:validation][:report_script_embeds]
+    when INVALID_TAG_MSG, INVALID_PREFIX
+      options[:validation][:report_invalid_tags]
+    when PARSE_ENTITY_REF
+      options[:validation][:report_missing_names]
+    when DOCTYPE_MSG
+      options[:validation][:report_missing_doctype]
+    else
+      true
     end
   end
 end
diff --git a/lib/html-proofer/check/links.rb b/lib/html-proofer/check/links.rb
index 8edb18f6..0862a69a 100644
--- a/lib/html-proofer/check/links.rb
+++ b/lib/html-proofer/check/links.rb
@@ -31,7 +31,7 @@ def run
       # is there even an href?
       if missing_href?
         # HTML5 allows dropping the href: http://git.io/vBX0z
-        next if @html.internal_subset.name == 'html' && @html.internal_subset.external_id.nil?
+        next if @html.internal_subset.nil? || (@html.internal_subset.name == 'html' && @html.internal_subset.external_id.nil?)
         add_issue('anchor has no href attribute', line: line, content: content)
         next
       end
diff --git a/lib/html-proofer/configuration.rb b/lib/html-proofer/configuration.rb
index 9348e2a3..8e8d9663 100644
--- a/lib/html-proofer/configuration.rb
+++ b/lib/html-proofer/configuration.rb
@@ -44,7 +44,8 @@ module Configuration
     VALIDATION_DEFAULTS = {
       :report_script_embeds => false,
       :report_missing_names => false,
-      :report_invalid_tags => false
+      :report_invalid_tags => false,
+      :report_missing_doctype => false
     }
 
     CACHE_DEFAULTS = {}
diff --git a/lib/html-proofer/utils.rb b/lib/html-proofer/utils.rb
index 450a7c6a..e1e72c52 100644
--- a/lib/html-proofer/utils.rb
+++ b/lib/html-proofer/utils.rb
@@ -1,4 +1,4 @@
-require 'nokogiri'
+require 'nokogumbo'
 
 module HTMLProofer
   module Utils
@@ -15,7 +15,7 @@ def create_nokogiri(path)
         content = path
       end
 
-      Nokogiri::HTML(clean_content(content))
+      Nokogiri::HTML5(content)
     end
     module_function :create_nokogiri
 
@@ -26,15 +26,5 @@ def swap(href, replacement)
       href
     end
     module_function :swap
-
-    # address a problem with Nokogiri's parsing URL entities
-    # problem from http://git.io/vBYU1
-    # solution from http://git.io/vBYUi
-    def clean_content(string)
-      string.gsub(%r{https?://([^>]+)}i) do |url|
-        url.gsub(/&(?!amp;)/, '&amp;')
-      end
-    end
-    module_function :clean_content
   end
 end
diff --git a/spec/html-proofer/command_spec.rb b/spec/html-proofer/command_spec.rb
index a4d81e0f..6e06878e 100644
--- a/spec/html-proofer/command_spec.rb
+++ b/spec/html-proofer/command_spec.rb
@@ -87,7 +87,7 @@
   end
 
   it 'works with check-html' do
-    broken = "#{FIXTURES_DIR}/html/invalid_tag.html"
+    broken = "#{FIXTURES_DIR}/html/unmatched_end_tag.html"
     output = make_bin('--check-html --report-invalid-tags', broken)
     expect(output).to match('1 failure')
   end
diff --git a/spec/html-proofer/element_spec.rb b/spec/html-proofer/element_spec.rb
index 474aafff..2d87f702 100644
--- a/spec/html-proofer/element_spec.rb
+++ b/spec/html-proofer/element_spec.rb
@@ -2,24 +2,24 @@
 
 describe HTMLProofer::Element do
   before(:each) do
-    @check = HTMLProofer::Check.new('', '', Nokogiri::HTML(''), HTMLProofer::Configuration::PROOFER_DEFAULTS)
+    @check = HTMLProofer::Check.new('', '', Nokogiri::HTML5(''), HTMLProofer::Configuration::PROOFER_DEFAULTS)
   end
 
   describe '#initialize' do
     it 'accepts the xmlns attribute' do
-      nokogiri = Nokogiri::HTML('<a xmlns:cc="http://creativecommons.org/ns#">Creative Commons</a>')
+      nokogiri = Nokogiri::HTML5('<a xmlns:cc="http://creativecommons.org/ns#">Creative Commons</a>')
       checkable = HTMLProofer::Element.new(nokogiri.css('a').first, @check)
       expect(checkable.instance_variable_get(:@xmlns_cc)).to eq 'http://creativecommons.org/ns#'
     end
 
     it 'assignes the text node' do
-      nokogiri = Nokogiri::HTML('<p>One')
+      nokogiri = Nokogiri::HTML5('<p>One')
       checkable = HTMLProofer::Element.new(nokogiri.css('p').first, @check)
       expect(checkable.instance_variable_get(:@text)).to eq 'One'
     end
 
     it 'accepts the content attribute' do
-      nokogiri = Nokogiri::HTML('<meta name="twitter:card" content="summary">')
+      nokogiri = Nokogiri::HTML5('<meta name="twitter:card" content="summary">')
       checkable = HTMLProofer::Element.new(nokogiri.css('meta').first, @check)
       expect(checkable.instance_variable_get(:@content)).to eq 'summary'
     end
@@ -27,13 +27,13 @@
 
   describe '#ignores_pattern_check' do
     it 'works for regex patterns' do
-      nokogiri = Nokogiri::HTML('<script src=/assets/main.js></script>')
+      nokogiri = Nokogiri::HTML5('<script src=/assets/main.js></script>')
       checkable = HTMLProofer::Element.new(nokogiri.css('script').first, @check)
       expect(checkable.ignores_pattern_check([/\/assets\/.*(js|css|png|svg)/])).to eq true
     end
 
     it 'works for string patterns' do
-      nokogiri = Nokogiri::HTML('<script src=/assets/main.js></script>')
+      nokogiri = Nokogiri::HTML5('<script src=/assets/main.js></script>')
       checkable = HTMLProofer::Element.new(nokogiri.css('script').first, @check)
       expect(checkable.ignores_pattern_check(['/assets/main.js'])).to eq true
     end
@@ -41,7 +41,7 @@
 
   describe '#url' do
     it 'works for src attributes' do
-      nokogiri = Nokogiri::HTML('<img src=image.png />')
+      nokogiri = Nokogiri::HTML5('<img src=image.png />')
       checkable = HTMLProofer::Element.new(nokogiri.css('img').first, @check)
       expect(checkable.url).to eq 'image.png'
     end
@@ -49,7 +49,7 @@
 
   describe '#ignore' do
     it 'works for twitter cards' do
-      nokogiri = Nokogiri::HTML('<meta name="twitter:url" data-proofer-ignore content="http://example.com/soon-to-be-published-url">')
+      nokogiri = Nokogiri::HTML5('<meta name="twitter:url" data-proofer-ignore content="http://example.com/soon-to-be-published-url">')
       checkable = HTMLProofer::Element.new(nokogiri.css('meta').first, @check)
       expect(checkable.ignore?).to eq true
     end
diff --git a/spec/html-proofer/fixtures/html/weird_onclick.html b/spec/html-proofer/fixtures/html/weird_onclick.html
index 3ca33457..ee58f094 100644
--- a/spec/html-proofer/fixtures/html/weird_onclick.html
+++ b/spec/html-proofer/fixtures/html/weird_onclick.html
@@ -1 +1 @@
-<a href="http://www.google.com" onClick="window.open('https://calendar.google.com/calendar/embed?src=o3uckm13gkcc5tu4u3846ik0pc%40group.calendar.google.com&ctz=America/Los_Angeles','Team Calendar','')">
+<a href="http://www.google.com" onClick="window.open('https://calendar.google.com/calendar/embed?src=o3uckm13gkcc5tu4u3846ik0pc%40group.calendar.google.com&ctz=America/Los_Angeles','Team Calendar','')"></a>
diff --git a/spec/html-proofer/html_spec.rb b/spec/html-proofer/html_spec.rb
index 98b3c92a..ac56e3da 100644
--- a/spec/html-proofer/html_spec.rb
+++ b/spec/html-proofer/html_spec.rb
@@ -19,40 +19,40 @@
     expect(proofer.failed_tests).to eq []
   end
 
-  it 'fails for an invalid tag' do
+  it 'works for custom tags' do
     html = "#{FIXTURES_DIR}/html/invalid_tag.html"
     proofer = run_proofer(html, :file, { :check_html => true, :validation => { :report_invalid_tags => true} })
-    expect(proofer.failed_tests.first).to match(/Tag myfancytag invalid \(line 2\)/)
+    expect(proofer.failed_tests).to eq []
   end
 
   it 'fails for an unmatched end tag' do
     html = "#{FIXTURES_DIR}/html/unmatched_end_tag.html"
     proofer = run_proofer(html, :file, { :check_html => true })
-    expect(proofer.failed_tests.first).to match(/Unexpected end tag : div \(line 3\)/)
+    expect(proofer.failed_tests.first).to include("@3:1: That tag isn't allowed here  Currently open tags: html, body..\n</div>\n^ (line 3)")
   end
 
-  it 'fails for an unescaped ampersand in attribute' do
+  it 'allows an unescaped ampersand in attribute' do
     html = "#{FIXTURES_DIR}/html/unescaped_ampersand_in_attribute.html"
     proofer = run_proofer(html, :file, { :check_html => true })
-    expect(proofer.failed_tests.first).to match(/htmlParseEntityRef: expecting ';' \(line 2\)/)
+    expect(proofer.failed_tests).to eq []
   end
 
   it 'fails for mismatch between opening and ending tag' do
     html = "#{FIXTURES_DIR}/html/opening_and_ending_tag_mismatch.html"
     proofer = run_proofer(html, :file, { :check_html => true })
-    expect(proofer.failed_tests.first).to match(/Opening and ending tag mismatch: p and strong/)
+    expect(proofer.failed_tests.first).to include("@3:31: That tag isn't allowed here  Currently open tags: html, body, p, strong..\n<p>The quick <strong>brown fox</p>\n                              ^ (line 3)")
   end
 
   it 'fails for div inside head' do
     html = "#{FIXTURES_DIR}/html/div_inside_head.html"
     proofer = run_proofer(html, :file, { :check_html => true })
-    expect(proofer.failed_tests.first).to match(/Unexpected end tag : head \(line 5\)/)
+    expect(proofer.failed_tests.first).to include("@5:1: That tag isn't allowed here  Currently open tags: html, body..\n</head>\n^ (line 5)")
   end
 
   it 'fails for missing closing quotation mark in href' do
     html = "#{FIXTURES_DIR}/html/missing_closing_quotes.html"
     proofer = run_proofer(html, :file, { :check_html => true })
-    expect(proofer.failed_tests.to_s).to match(/Couldn't find end of Start Tag a \(line 6\)/)
+    expect(proofer.failed_tests.to_s).to include("@6:1: Tokenizer error with an unimplemented error message.\\n\\n^ (line 6)\"]")
   end
 
   it "doesn't fail for single ampersand" do
@@ -61,10 +61,10 @@
     expect(proofer.failed_tests).to eq []
   end
 
-  it "fails for single ampersand when asked" do
+  it "allows single ampersand" do
     html = "#{FIXTURES_DIR}/html/single_amp.html"
     proofer = run_proofer(html, :file, { :check_html => true, :validation => { :report_missing_names => true } })
-    expect(proofer.failed_tests.first).to match('htmlParseEntityRef: no name')
+    expect(proofer.failed_tests).to eq []
   end
 
   it 'ignores embeded scripts when asked' do
@@ -78,7 +78,7 @@
     opts = { :check_html => true, :validation => { :report_script_embeds => true } }
     ignorableScript = "#{FIXTURES_DIR}/html/ignore_script_embeds.html"
     proofer = run_proofer(ignorableScript, :file, opts)
-    expect(proofer.failed_tests.length).to eq 2
+    expect(proofer.failed_tests).to eq []
   end
 
   it 'does not fail for weird iframe sources' do
diff --git a/spec/html-proofer/links_spec.rb b/spec/html-proofer/links_spec.rb
index 164b4e29..aa6dc1fa 100644
--- a/spec/html-proofer/links_spec.rb
+++ b/spec/html-proofer/links_spec.rb
@@ -64,10 +64,10 @@
     expect(proofer.failed_tests.first).to match(/internally linking to .\/notreal.html, which does not exist/)
   end
 
-  it 'fails for link with no href' do
+  it 'allows link with no href' do
     missingLinkHrefFilepath = "#{FIXTURES_DIR}/links/missingLinkHref.html"
     proofer = run_proofer(missingLinkHrefFilepath, :file)
-    expect(proofer.failed_tests.first).to match(/anchor has no href attribute/)
+    expect(proofer.failed_tests).to eq []
   end
 
   it 'should follow redirects' do
@@ -208,16 +208,16 @@
     expect(proofer.failed_tests).to eq []
   end
 
-  it 'fails for empty href within link elements' do
+  it 'allows empty href on link elements' do
     head_link = "#{FIXTURES_DIR}/links/head_link_href_empty.html"
     proofer = run_proofer(head_link, :file)
-    expect(proofer.failed_tests.first).to match(/anchor has no href attribute/)
+    expect(proofer.failed_tests).to eq []
   end
 
-  it 'fails for absent href within link elements' do
+  it 'allows missing href on link elements' do
     head_link = "#{FIXTURES_DIR}/links/head_link_href_absent.html"
     proofer = run_proofer(head_link, :file)
-    expect(proofer.failed_tests.first).to match(/anchor has no href attribute/)
+    expect(proofer.failed_tests).to eq []
   end
 
   it 'fails for internal linking to a directory without trailing slash' do
@@ -465,7 +465,7 @@
 
     missing_href = "#{FIXTURES_DIR}/links/blank_href_htmlunknown.html"
     proofer = run_proofer(missing_href, :file)
-    expect(proofer.failed_tests.length).to eq 1
+    expect(proofer.failed_tests).to eq []
   end
 
   it 'works with internal_domains' do
diff --git a/spec/html-proofer/scripts_spec.rb b/spec/html-proofer/scripts_spec.rb
index b48ce7d1..b752587e 100644
--- a/spec/html-proofer/scripts_spec.rb
+++ b/spec/html-proofer/scripts_spec.rb
@@ -16,7 +16,8 @@
   it 'fails for missing internal src' do
     file = "#{FIXTURES_DIR}/scripts/script_missing_internal.html"
     proofer = run_proofer(file, :file)
-    expect(proofer.failed_tests.first).to match(/doesnotexist.js does not exist \(line 5\)/)
+    expect(proofer.failed_tests.length).to eq 1
+    expect(proofer.failed_tests.first).to include('spec/html-proofer/fixtures/scripts/script_missing_internal.html: internal script doesnotexist.js does not exist (line 5)')
   end
 
   it 'works for present content' do