CSS Flexbox can create a disconnect between the DOM order and visual presentation of content, causing keyboard navigation to break. For this reason, the CSS Flexible Box Layout module warns against resequencing content logic, but asking authors not to use flexbox in this way seems illogical in itself.
The CSS Speech properties are intended to give content aural style, in the same way other CSS properties give content visual style. The CSS Speech module is unsupported by browsers, but the Web Speech API can be used to demonstrate something of the way CSS Speech might work in practice.
The CSS before/after pseudo-selectors can be used to insert content into a page. In some situations this technique is a useful thing to do, but how do browsers and screen readers handle the generated content?