<input type="zoom" step="n"> step attribute should specify how base min value used in step calculation is found

[prushforth]

Currently the spec for step reads:

The increment by which a value may vary between the min and max values.

but doesn't specify how the min value used for step calculation is derived. The spec should say that the base min value is calculated according to: the min attribute if present; a <meta name="zoom" content="...">; default TCRS min (0).