In a flex container with a text child (such as a label) and an <input type=“text”> child, baseline alignment works as expected only if the input element has a value specified. If the input element has no value, the sibling text element will be aligned to the bottom of the text input widget, rather than where its text baseline would be.
(The testcase uses the unprefixed syntax, but the issue is the same whether using r181845 with no prefixes, or the shipping Safari 8.0.4 (10600.4.10.7) with prefixes.)
See also http://crbug.com/401185 for the Blink issue. Firefox renders the testcase correctly.
Also possibly related: https://bugs.webkit.org/show_bug.cgi?id=35787
This is still a big problem in Safari 13.
If there is text or a placeholder in the field, it isn't an issue. Only when the field is completely blank (you can enter text into it and the tab out to see the alignment jump). A placeholder of a non-breaking space kind of works, but it still doesn't align as well as an actual letter (baseline on a space seem to be different).