是否有一些规范说浏览器应该只处理JavaScript相关的MIME类型？

请参阅 type attribute ：

这个属性给出关于链接目标地址处可用内容的内容类型的建议性提示。它允许用户代理选择使用回退机制，而不是获取内容，如果他们被告知将获得他们不支持的内容类型的内容。

如果您想要获取脚本中使用的任意内容，请使用XMLHttpRequest。

It appears this code will request the file in Chrome and IE but not in Firefox.

<script type="text/my-custom-mime-type" src="test.ashx">
</script>

Is there a some spec that says browsers should only process JavaScript related mime-types? I know IE probably supports this because of the history with vbscript.

Once you have "content" like this downloaded how can you get access to it? Does JavaScript/jQuery/? have some way of getting at this.

UPDATE So there is 2 parts to question. Sounds like for the first part - the browser will download what it will download and I guess there isn't much you can do about that based off the answers so far.

Example:
<script type="text/xml-script">
<page xmlns="http://schemas.microsoft.com/xml-script/2005"> <components>
<application load="page_load" />
</components>
</page>
</script>
</pre>

this is a snippet from Microsoft's declarative MSAjax tech. Could you pull this in from an external file. Note: I'm not trying to use MSAjax here, but its a good example of a custom type being used for a script tag.

Part 2 - can you get access to the text if the "content" does download? For example, lets say its JavaScript - could you display it in a textbox? (without an explicit Ajax call)?

Is there a some spec that says browsers should only process JavaScript related mime-types?

See the type attribute:

This attribute gives an advisory hint as to the content type of the content available at the link target address. It allows user agents to opt to use a fallback mechanism rather than fetch the content if they are advised that they will get content in a content type they do not support.

If you want to fetch arbitrary content for use in a script, use XMLHttpRequest.