I was using Mask 2.1.1 with Typo3 7.6.11 for a site as described in the Mask manual with this typoscript:

page < temp.mask.page
page.10.file.stdWrap.cObject.default.value = fileadmin/templates/index.html
page {
        includeCSS.styles = fileadmin/templates/style.css
}

The problem with this was that the CSS and JS files includes from extensions where missing, even though the static templates of this where included. The reason is that this typoscript overwrites the page.include* stuff defined by the templates.

A workaround

To solve this, I copied the page.include* stuff defined by the extensions in a temporary variable and restored it after loading the mask object into page:

tempJSFooterlibs < page.includeJSFooterlibs
tempJSFooter < page.includeJSFooter
tempCSS < page.includeCSS

page < temp.mask.page
page.10.file.stdWrap.cObject.default.value = fileadmin/templates/index.html
page {
        includeJSFooterlibs < tempJSFooterlibs
        includeJSFooter < tempJSFooter
        includeCSS < tempCSS

        includeCSS.styles = fileadmin/templates/style.css
}

There are more includes you might need to consider if your extensions use these:

page.includeJS
page.includeJSlibs
page.jsInline
page.jsFooterInline

Hope this helps someone with the same problem! Please let me know if there is an easier solution.

Update 06.07.2017: The clean way

So the workaround described above works, but it has a disadvantage: You might install a plugin that defines some other page.* property that gets lost. For example, I just had this problem with mindshape_cookie_hint: It defines page.9877 which gets lost when you overwrite page with temp.mask.page. It took quite some time until I found the reason for this problem. And then I found another way which is much simpler than the one described before:

page = PAGE
page.10 < temp.mask.page.10

Just don’t copy the whole temp.mask.page, only temp.mask.page.10. This will preserve all the stuff defined by plugins. The solution is so simple and easy!

So I think this is not really a bug in mask, it is rather a mistake in the documentation.

Please let me know in the comments if this saved your day or if you have suggestions.