This issue is applicable to all languages that are written vertically.

All boxes used for form input should contain vertical text, should be rotated so that they progress down the page, and their content should progress in vertical lines from left to right. See User interaction in the requirements document. This is particularly important for Mongolian, among vertically set orthographies, because horizontal lines don't work.

Specs:
css-writing-modes-3 specifies the requirements for vertical text support, including examples of form controls rendered to match the writing mode.

Tests & results:
i18n test suite, Forms
Only Gecko vertically orients forms, but in pull-down select boxes it doesn't vertically orientate the selection options. Blink and Webkit fail to orient forms vertically at all. (The tests use Japanese text and don't check whether lines flow left to right for Mongolian.)

Here are some other tests, again in Japanese: standard syntax • proprietary syntax

Browser bug reports:
WebKit. The WebKit source has

358 /* Form controls don't go vertical. */
359 input, textarea, keygen, select, button, meter, progress {
360 -webkit-writing-mode: horizontal-tb !important;
361 }

Priority:
This has a significant impact for creation of general web sites.

For more details, see this GitHub issue, which is being used to track this gap. Please add any discussion there, and not to this issue.