@polesye I think we should either use ```innerHTML``` or ```document.createTextNode``` but not a mix of the two. Perhaps createTextNode is safer.

http://stackoverflow.com/questions/17195868/what-is-a-text-node-its-uses-document-createtextnode

How about:
```
this.video.appendChild(document.createTextNode(_.values(sourceStr).join('')));
errorMessage = document.createTextNode(gettext(
    'This browser cannot play .mp4, .ogg, or ' +
    '.webm files. Try using a different browser, such as Google ' +
    'Chrome.'
this.video.appendChild(errorMessage);
```