Looks good to me. 👍

@Maks3w doesn't seem like there's any BC break here - can be thrown at develop IMO :D

@heiglandreas #5623 provides some logic to ensure that

1. legends will only be rendered if the fieldset has a label defined
2. fieldsets will render attributes as set on the collection

On merging your PR to the develop branch, I had a merge conflict due to those changes. I tried changing the $wrapper to read '<fieldset%s>%2$s%1$s%3$s</fieldset>', and altering the render() logic to inject the attributes, if any. However, this results in practically all of your new tests failing.

Any chance you can take a look at it?

I tried changing the $wrapper to read '<fieldset%s>%2$s%1$s%3$s',

This was wrong, once I paid more attention to the sprintf() invocation. I altered it as follows:

    '<fieldset%1$s>%3$s%2$s%4$s</fieldset>'

And the sprintf() invocation reads:

$markup = sprintf(
    $this->wrapper,
    $attributeString,
    $markup,
    $legend,
    $templateMarkup
);

This gets me closer - now only 2 failing tests. Will let you know if I get any closer.

@heiglandreas Got it! Merged to develop for release with 2.3.0.