Showing captions on local drive in Webkit

Hi All,

I'm getting an error in Webkit browsers when I try to add WebVTT captions
to an HTML5 video on a local drive. Here's the code:

<video controls>  <source src="test.mp4" type="video/mp4">  <source
src="test.webm" type="video/webm">  <track kind="captions"
src="test.vtt" srclang="en" default></video>

All test.* files are located in the same local directory as the HTML file.
The caption file validates according to https://quuz.org/webvtt/
Local captions play fine in both Firefox and IE11.
However, in Chrome, Safari, and Opera the video plays fine but there are no
captions. The following error appears in the console:

> Text track from origin 'file://' has been blocked from loading: Not at
same origin as the document, and parent of track element does not have a
'crossorigin' attribute. Origin 'null' is therefore not allowed access.

The captions play fine in these same Webkit browsers if all content is
hosted on a web server.

I'm assuming this is a bug since the assertion "Not at same origin as the
document" is false. Is there any reason to think otherwise? Do local WebVTT
files pose any security risks that WebVTT files on a web server don't? Does
the crossorigin attribute really have any role in delivering captions?

My use case is teaching students to add captions in a web design course.
They don't have convenient access to a web server but they can add HTML5
video to a web page on their local drive and instantly view the results in
any browser. However, unless there's a workaround they apparently can't do
the same with captions.

Thanks,
Terrill


---
Terrill Thompson
Technology Accessibility Specialist
DO-IT, Accessible Technology Services
UW Information Technology
University of Washington
tft@uw.edu