title: Cannot find the declaration of element 'android.support.constraint.ConstraintLayout'
body: <p>I have created an xml file called <code>activity_main(sw50dp)</code>, but when I try to validate it, it gives me the errors:</p>
  
  <blockquote>
    <p>Error:(4, 42) cvc-elt.1.a: Cannot find the declaration of element 'android.support.constraint.ConstraintLayout'.</p>
  </blockquote>
  
  <p>and</p>
  
  <pre><code>C:\Program Files\Android\Android Studio\lib\idea.jar!\standardSchemas\xhtml1.dtd
  
  Error:(26, 3) The markup in the document preceding the root element must be well-formed.
  </code></pre>
  
  <p>activity_main file</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;android.support.constraint.ConstraintLayout 
  xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:app="http://schemas.android.com/apk/res-auto"
  xmlns:tools="http://schemas.android.com/tools"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:background="@drawable/images"
  tools:context=".MainActivity"&gt;
  
  &lt;Button
      android:id="@+id/buttonDiv"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:layout_marginStart="320dp"
      android:layout_marginTop="340dp"
      android:background="@drawable/circle"
      android:text="÷"
      app:layout_constraintEnd_toEndOf="parent"
      app:layout_constraintRight_toRightOf="parent"
      app:layout_constraintTop_toTopOf="parent" /&gt;
  
  &lt;Button
  
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:layout_marginEnd="220dp"
      android:layout_marginStart="100dp"
      android:background="@drawable/oval"
      android:text="2"
      app:layout_constraintLeft_toRightOf="@+id/button1"
      app:layout_constraintStart_toEndOf="@+id/button1"
      app:layout_constraintTop_toTopOf="@+id/button1" /&gt;
  
  &lt;Button
      android:id="@+id/button1"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:layout_marginEnd="320dp"
      android:layout_marginRight="0dp"
      android:layout_marginTop="118dp"
      android:background="@drawable/oval"
      android:text="1"
      app:layout_constraintLeft_toLeftOf="parent"
      app:layout_constraintStart_toStartOf="parent"
      app:layout_constraintTop_toTopOf="parent" /&gt;
  
  &lt;Button
      android:id="@+id/buttonReset"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:layout_marginEnd="320dp"
      android:layout_marginRight="0dp"
      android:layout_marginTop="45dp"
      android:background="@drawable/oval"
      android:text="C"
      app:layout_constraintLeft_toLeftOf="parent"
      app:layout_constraintStart_toStartOf="parent"
      app:layout_constraintTop_toTopOf="parent" /&gt;
  
  &lt;TextView
      android:id="@+id/textNum"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:layout_marginEnd="100dp"
      android:layout_marginStart="100dp"
  
  
  &lt;Button
      android:id="@+id/button5"
      android:layout_width="match_parent"
      android:layout_height="wrap_content"
      android:layout_marginEnd="220dp"
      android:layout_marginStart="100dp"
      android:background="@drawable/oval"
  
  
  &lt;/android.support.constraint.ConstraintLayout&gt;
  </code></pre>
  
  <p>activity_main(sw50dp) file</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;android.support.constraint.ConstraintLayout
  xmlns:android="http://schemas.android.com/apk/res/android"
  android:layout_width="match_parent"
  android:layout_height="match_parent"&gt;
  </code></pre>
  
  <p>I have created other layout files as well for example activity_main(normal-land). I didn't have any problems with them.</p>