Fix HTML validation (mastodon#10354)

* Fix HTML validation

* Report first HTML error instead on validation error

Author: ClearlyClaire

app/validators/html_validator.rb

@@ -3,12 +3,16 @@
 class HtmlValidator < ActiveModel::EachValidator
   def validate_each(record, attribute, value)
     return if value.blank?
-    record.errors.add(attribute, I18n.t('html_validator.invalid_markup')) unless valid_html?(value)
+    errors = html_errors(value)
+    unless errors.empty?
+      record.errors.add(attribute, I18n.t('html_validator.invalid_markup', error: errors.first.to_s))
+    end
   end
 
   private
 
-  def valid_html?(str)
-    Nokogiri::HTML.fragment(str).to_s == str
+  def html_errors(str)
+    fragment = Nokogiri::HTML.fragment(str)
+    fragment.errors
   end
 end

config/locales/en.yml

@@ -638,7 +638,7 @@ en:
       one: Something isn't quite right yet! Please review the error below
       other: Something isn't quite right yet! Please review %{count} errors below
   html_validator:
-    invalid_markup: contains invalid HTML markup
+    invalid_markup: 'contains invalid HTML markup: %{error}'
   identity_proofs:
     active: Active
     authorize: Yes, authorize