title: Show HTML5 <video> on previewHtml command in vscode extension
body: <p>I'm trying to write a new simple extension that shows an mp4 video player using <code>&lt;video&gt;</code>. Vscode seems to support video playback because I can watch an YouTube video perfectly.</p>
  
  <p>But I can't make my own mp4 work. It simply doesn't play.</p>
  
  <p>I suspect this has something to do with cross origin requests. But I don't know how to debug Network requests inside vscode html preview. Or maybe it's entirely different.</p>
  
  <hr>
  
  <p>The code in <code>extension.ts</code> is very simple:</p>
  
  <pre class="lang-ts prettyprint-override"><code>'use strict';
  import * as vscode from 'vscode';
  
  export function activate(context: vscode.ExtensionContext) {
      let uri = vscode.Uri.parse('video-player://authority/play');
  
      class TextDocumentContentProvider implements vscode.TextDocumentContentProvider {
          private _onDidChange = new vscode.EventEmitter&lt;vscode.Uri&gt;();
  
          public provideTextDocumentContent(uri: vscode.Uri): string {
              return `
                  &lt;video crossorigin="anonymous" controls src="https://example.com/video.mp4"&gt;&lt;/video&gt;
                  `;
          }
  
          get onDidChange(): vscode.Event&lt;vscode.Uri&gt; {
              return this._onDidChange.event;
          }
  
          public update(uri: vscode.Uri) {
              this._onDidChange.fire(uri);
          }
  
      }
  
      let provider = new TextDocumentContentProvider();
      let registration = vscode.workspace.registerTextDocumentContentProvider('video-player', provider);
  
      let disposable = vscode.commands.registerCommand('extension.playVideo', () =&gt; {
          return vscode.commands.executeCommand('vscode.previewHtml', uri, vscode.ViewColumn.Two, 'Video Player').then((success) =&gt; {
          }, (reason) =&gt; {
              vscode.window.showErrorMessage(reason);
          });
      });
  
      context.subscriptions.push(disposable, registration);
  }
  </code></pre>