[Hint][Flexbox] Report Some HTML elements can't be flex containers

[captainbrosset]

(from https://github.com/philipwalton/flexbugs#flexbug-9)

Certain HTML elements, like <summary>, <fieldset> and <button>, do not work as flex containers. The browser's default rendering of those element's UI conflicts with the display: flex declaration.

Many browsers used to suffer from this bug, to various levels, but got fixed in recent times.
The issue described in https://github.com/philipwalton/flexbugs#flexbug-9 is no longer up to date I believe.
Some problems persist: fieldset elements can't be flex containers in Edge release (85 at the time of writing) but this got fixed in Edge Canary (87 at the time of writing).

So, to be investigated more closely whether a hint that warns users about this is actually needed or not.

[captainbrosset]

As said above, these bugs got fixed very recently in major browsers. In 6 weeks from now, it will become possible to use a fieldset as a flex container in Edge and Chrome standard releases.
It might be worth the effort to write a hint.

However, @antross brought up a good point that certain older versions of chrome still exist out there in the wild. Chrome 49, which was the last compatible version on Windows XP. And another chrome version on Android which I can't remember the exact details.

So, a hint that warns about this might still be in order.