How should rounding errors in step constraint validation be treated?

[teapotd]

Spec for step constraint validation says:

Constraint validation: When the element has an allowed value step, and the result of applying the algorithm to convert a string to a number to the string given by the element's value is a number, and that number subtracted from the step base is not an integral multiple of the allowed value step, the element is suffering from a step mismatch.

input[type=number]'s algorithm to convert a string to a number follows rules for parsing floating-point number values which specifically mention rounding to IEEE 754 double-precision float:

15. Conversion: Let S be the set of finite IEEE 754 double-precision floating-point values except −0, but with two special values added: 2^1024 and −2^1024.
16. Let rounded-value be the number in S that is closest to value, selecting the number with an even significand if there are two equally close values. (The two special values 2^1024 and −2^1024 are considered to have even significands for this purpose.)
17. If rounded-value is 2^1024 or −2^1024, return an error.
18. Return rounded-value.

step attribute value is also parsed using these steps. It implies that step mismatch check should check if one base-2 floating-point number is exact integer multiple of another, which doesn't make much sense in practice.

Firefox and Chrome use decimal floating-point numbers instead to avoid rounding.

[TimothyGu]

Here's an even more interesting case, to stress test stepping.

<input type=number id=input min="-100000000" step="1e-12" value=0>
<script>
  const LONG_MAX = 2147483647; // maximum value of long IDL type
  input.stepUp(LONG_MAX);
  console.log(input.value);
</script>

Even though Firefox and Chrome both use some high-precision decimal library, their output is different: Chrome outputs 0, while Firefox outputs 0.002147483647. I checked with jsdom that even if a naïve floating point operation were to be used, it should still be 0.002147483647. Not sure if this is a bug in Chrome or Firefox or jsdom.