This page is testing what IE8 probably ought to do to be consistent with its intentions. (This is not what any specification says a browser must do, so there is no expectation that other browsers will pass these tests.)

<object data="resources/html_as_text_plain" type="text/html"></object>

There should be a line saying "This is text/html" here:

<object data="resources/html_as_text_plain_nosniff" type="text/html"></object>

There should be a line saying roughly "This is text/plain" here:

<img src="resources/png_as_text_plain">

There should be an image saying "Loaded PNG image" here:

<img src="resources/png_as_text_plain_nosniff">

There should not be an image here:

<object data="resources/png_as_text_plain" type="image/jpeg"></object>

There should be an image saying "Loaded PNG image" here:

<object data="resources/png_as_text_plain_nosniff" type="image/jpeg"></object>

There should not be an image here:

<script src="resources/script_as_text_plain"></script>

There should be a string saying "Loaded script" here:

<script src="resources/script_as_text_plain_nosniff"></script>

There should not be a string saying "Loaded script" here:

<link rel="stylesheet" href="resources/css_as_text_plain">

This text should have a green background.

<link rel="stylesheet" href="resources/css_as_text_plain_nosniff">

This text should have a green background.