intellij-plugin/features/code-insight-html/resources/code-insight-html.xml

<idea-plugin>
  <dependencies>
    <!--suppress PluginXmlValidity -->
    <module name="code-insight-core"/>
    <plugin id="com.intellij.modules.xml"/>
  </dependencies>

  <extensions defaultExtensionNs="com.intellij">
    <completion.contributor language="HTML"
                            id="EduHtmlCompletionContributor"
                            implementationClass="com.jetbrains.edu.html.taskDescription.EduHtmlCompletionContributor"/>
    <psi.referenceContributor language="HTML"
                              implementation="com.jetbrains.edu.html.taskDescription.EduHtmlReferenceContributor"/>
  </extensions>
</idea-plugin>