If the background-color of this <div> is

Lime:

the empty-string inside <meta http-equiv="default-style content="<the-empty-string>">, does indeed does either mean that <style title="<the-empty-string>"> is treated like the preferred stylesheet or it means that any empty title makes the stylesheet be treated like a persistent stylesheet. Actually, the latter seems to be the case.

UAs with this behaviour: none.

Orange:

the emptystring inside a not a single meta default-style declaration is not respected used – the last meta default-style declaration preferred stylesheet is used instead.

UAs with this behaviour: Opera, IE6, IE8.

Red:

the emptystring inside a meta default-style declaration is not respected used – instead the preceding meta default-style declaration is used.

UAs with this behaviour: Webkit (Safari 4), Chrome, Konqueror, Mozillas (e.g. Firefox 3.6)

If the background-color of this <div> is

Lime:

the emptystring of the last occuring meta content-language declaration is respected, and the language of the document is set to unknown.

UAs with this behaviour: Webkit (Safari 4), Chrome, Konqueror, Mozillas (e.g. Firefox 3.6), Opera, IE6, IE8.

Red:

the emptystring of the last occuring meta content-language declaration is not respected, and the Web browser has instead visited the earlier/preceding meta content-language declaration.

UAs with this behaviour: Mozilla browsers (only)