Saxon

Firstly, it's great to know that Altova are studying and implementing the spec. I just wish they would participate in its development, if only by providing feedback when they find issues. Contributing test cases would be public-spirited too...

Secondly, I may be missing it, but I don't see a rule saying that the default mode must be have public visibility. On the contrary, we explicitly say that the default mode can be the unnamed mode, and the unnamed mode always has private visibility.

So I would say that your code is valid.

Thanks for your response, I have now searched the spec and https://www.w3.org/XML/Group/qtspecs/specifications/xslt-30/html/#dt-unnamed-mode has a table that says about the visibility of modes

visibility One of public, private, or final. The default is private. See 3.5.3.1 Visibility of Components. If the mode is unnamed, that is, if the name attribute is absent, then the visibility attribute if present must have the value private. A named mode is not eligible to be used as the initial mode if its visibility is private.

so it seems that Altova is right that the initial mode can't be a private mode and as the visibility of @m1@ in the using package is private without an @xsl:accept@ or @xsl:override@ it appears to be that Saxon should raise an error on the stylesheet trying to use the mode @m1@ as the @default-mode@ without doing an @xsl:accept@.

There's actually still some debate about some of the rules in this area, so I won't be making any changes until that is bedded down.

But the statement

A named mode is not eligible to be used as the initial mode if its visibility is private.

doesn't imply

A named mode is not eligible to be used as the default mode if its visibility is private.

The default mode in xsl:stylesheet is primarily a default value for the mode attribute of xsl:template and xsl:apply-templates. If you have a stylesheet module that's concerned with generating a table-of-contents, and most of the template rules are in mode TOC, then it makes sense to set default-mode="TOC" and leave the mode attribute absent. It might also make perfect sense to make this mode private, so that the only way of invoking the TOC functionality is through some public interface like a make-table-of-contents() function.

We do say: If no initial mode is supplied, then the mode used is that named in the default-mode attribute of the (explicit or implicit) xsl:package element of the top-level package or in the absence of such an attribute, the unnamed mode.] which implies that using a private mode as the default mode in the top-level package is probably a bad idea. This is actually the topic currently open in the WG: see bug 29827

https://www.w3.org/Bugs/Public/show_bug.cgi?id=29827