<defs> is specified as display:none in UA stylesheet.

[faceless2]

<defs> is still specified in the UA Stylesheet as "display:none", and that's matched in the prose.

But, given the resolution in w3c/fxtf-drafts#245 and w3c/csswg-drafts#4213, setting it to display:none will disable any filters, masks etc. contained within it. Obviously that's not desired. <defs> is not rendered, but it should be noted that it's unrelated to the display property, and the UA stylesheet shouldn't set it to none.

EDIT: same applies to mask and clipPath, which are also display:none in the UA stylesheet

(Originally posted in error to w3c/fxtf-drafts#404)