datetime-local value sanitization removes leading zeroes for seconds

[teapotd]

<input type="datetime-local" value="2000-01-01T12:30:00.5">
<input type="datetime-local" value="2000-01-01T12:30:01">
<script>
    let result = '';
    for (let elem of document.querySelectorAll("input")) {
        result += elem.valueAsNumber + ' ' + elem.value + '\n';
    }
    alert(result);
</script>

In the above example the leading zeroes for seconds are removed during sanitization, but they are required in valid datetime-local strings. This makes valueAsNumber fail, for example.

https://html.spec.whatwg.org/multipage/#valid-normalised-local-date-and-time-string
https://html.spec.whatwg.org/multipage/#valid-time-string

servo/components/script/dom/bindings/str.rs

Lines 469 to 484 in 63cb13f

	pub fn convert_valid_normalized_local_date_and_time_string(&mut self) -> Result<(), ()> {
	let ((year, month, day), (hour, minute, second)) =
	self.parse_local_date_and_time_string()?;
	if second == 0.0 {
	self.0 = format!(
	"{:04}-{:02}-{:02}T{:02}:{:02}",
	year, month, day, hour, minute
	);
	} else {
	self.0 = format!(
	"{:04}-{:02}-{:02}T{:02}:{:02}:{}",
	year, month, day, hour, minute, second
	);
	}
	Ok(())
	}

[pshaughn]

Yes, the last field there should also have {:02}, sorry I didn't catch this when I was otherwise tweaking the time string code.

[pshaughn]

{:02} is still wrong when there are millis; I've written a test to flex the various cases and I think I can pass them all by adding a branch for 0.0<second<10.0 that adds one leading zero