Interestingly, a documentation overview says that tb-rl is an allowed value: https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/Presentation#attr-writi

Interestingly, a documentation overview says that tb-rl is an allowed value: https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/Presentation#attr-writing-mode https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/P... But the detailed documentation it links to does not: https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/writing-mode https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/w... I had a look in Bugzilla and couldn't find anything specific to tb-rl, but did find an ancient meta bug for writing-mode which links to millions of other bugs: https://bugzilla.mozilla.org/show_bug.cgi?id=writing-mode https://bugzilla.mozilla.org/show_bug.cgi?id=writing-mode