(Perhaps I should have specified "Quantum" rather than "Gecko"? I meant the engine that new Firefox uses, I thought it was still called Gecko but sounds like I might be wrong?)

www.just-eat.no

Many (all?) Oslo postcodes start with 0, and their site truncates the 0 when you try to enter your postcode. Their stated solution was to use Chrome, so they don't get any of my business.

From your description, it sounds like they decided to use a "number" typed field for the post codes. This is of course incorrect, post codes are really just strings. While the bug manifests itself on firefox, it is hard to blame it for implementing the field according to spec [1]. Chrome decided to ignore the spec and leave the field more free-form, which in turn enables developers to use it incorrectly without detecting the compatibility bug.

[1]: https://www.w3.org/TR/html52/sec-forms.html#number-state-typ...

> User agents must not allow the user to set the value to a non-empty string that is not a valid floating-point number. If the user agent provides a user interface for selecting a number, then the value must be set to the best representation of the number representing the user’s selection as a floating-point number.

Notably, this type of issue is exactly why it's important to have more than one implementation of web standards!

If Chrome is your only target, the standard ceases to mean anything. The real standard is just whatever Chrome does.