diff --git a/CHANGELOG.md b/CHANGELOG.md
index 9493df10..7960026c 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -1,3 +1,7 @@
+## Unreleased
+
+* Fix a bug which resulted in validation errors on 'Start Button' elements [#237](https://github.com/alphagov/govspeak/pull/237)
+
 ## 6.8.0
 
 * Drop support for Ruby 2.6 which reaches End of Life (EOL) on 31/03/2022
diff --git a/lib/govspeak.rb b/lib/govspeak.rb
index be7bf9e0..134ab2a2 100644
--- a/lib/govspeak.rb
+++ b/lib/govspeak.rb
@@ -54,6 +54,7 @@ def initialize(source, options = {})
 
       @images = options.delete(:images) || []
       @allowed_elements = options.delete(:allowed_elements) || []
+      @allowed_image_hosts = options.delete(:allowed_image_hosts) || []
       @attachments = Array.wrap(options.delete(:attachments))
       @links = Array.wrap(options.delete(:links))
       @contacts = Array.wrap(options.delete(:contacts))
@@ -69,7 +70,8 @@ def initialize(source, options = {})
     def to_html
       @to_html ||= begin
         html = if @options[:sanitize]
-                 HtmlSanitizer.new(kramdown_doc.to_html).sanitize(allowed_elements: @allowed_elements)
+                 HtmlSanitizer.new(kramdown_doc.to_html, allowed_image_hosts: @allowed_image_hosts)
+                              .sanitize(allowed_elements: @allowed_elements)
                else
                  kramdown_doc.to_html
                end
diff --git a/lib/govspeak/html_sanitizer.rb b/lib/govspeak/html_sanitizer.rb
index edaadd52..9458f45e 100644
--- a/lib/govspeak/html_sanitizer.rb
+++ b/lib/govspeak/html_sanitizer.rb
@@ -46,7 +46,17 @@ def sanitize(allowed_elements: [])
       transformers << ImageSourceWhitelister.new(@allowed_image_hosts)
     end
 
-    Sanitize.clean(@dirty_html, Sanitize::Config.merge(sanitize_config(allowed_elements: allowed_elements), transformers: transformers))
+    # It would be cleaner to move this `transformers` key into the `sanitize_config` method rather
+    # than having to use Sanitize::Config.merge() twice in succession. However, `sanitize_config`
+    # is a public method and it looks like other projects depend on it behaving the way it
+    # currently does – i.e. to return Sanitize config without any transformers.
+    # e.g. https://github.com/alphagov/hmrc-manuals-api/blob/4a83f78d0bb839520155623fd9b63b3b12a3b13a/app/validators/no_dangerous_html_in_text_fields_validator.rb#L44
+    config_with_transformers = Sanitize::Config.merge(
+      sanitize_config(allowed_elements: allowed_elements),
+      transformers: transformers,
+    )
+
+    Sanitize.clean(@dirty_html, config_with_transformers)
   end
 
   def sanitize_config(allowed_elements: [])
diff --git a/lib/govspeak/html_validator.rb b/lib/govspeak/html_validator.rb
index 7fdada95..2e73fe43 100644
--- a/lib/govspeak/html_validator.rb
+++ b/lib/govspeak/html_validator.rb
@@ -1,9 +1,9 @@
 class Govspeak::HtmlValidator
   attr_reader :govspeak_string
 
-  def initialize(govspeak_string, sanitization_options = {})
+  def initialize(govspeak_string, options = {})
     @govspeak_string = govspeak_string.dup.force_encoding(Encoding::UTF_8)
-    @sanitization_options = sanitization_options
+    @allowed_image_hosts = options[:allowed_image_hosts]
   end
 
   def invalid?
@@ -11,17 +11,23 @@ def invalid?
   end
 
   def valid?
-    dirty_html = govspeak_to_html
-    clean_html = Govspeak::HtmlSanitizer.new(dirty_html, @sanitization_options).sanitize
+    dirty_html = govspeak_to_html(sanitize: false)
+    clean_html = govspeak_to_html(sanitize: true)
     normalise_html(dirty_html) == normalise_html(clean_html)
   end
 
+private
+
   # Make whitespace in html tags consistent
   def normalise_html(html)
     Nokogiri::HTML5.fragment(html).to_s
   end
 
-  def govspeak_to_html
-    Govspeak::Document.new(govspeak_string, sanitize: false).to_html
+  def govspeak_to_html(sanitize:)
+    Govspeak::Document.new(
+      govspeak_string,
+      sanitize: sanitize,
+      allowed_image_hosts: @allowed_image_hosts,
+    ).to_html
   end
 end