[edit]*

As you can see, there is no error in this code, my initial problem is due to the improper installation of Firefox on my computer. I didn't know it when I asked this question.

* I can't delete it (due to one answer with upvotes)

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this is what I would like to get:

┌─────┐┌─────┐│  A  │││└─────┘││┌─────┐│  B  ││  C  │││└─────┘││└─────┘

what I have done:

form {  display        : flex;  flex-direction : column;  flex-wrap      : wrap;  height         : 14em;  }fieldset {   display : block;  margin  : 1em .6em;  }fieldset:nth-child(2) {  order      : 2;  flex-basis : 10em;  }

<form action="" id="my-form"><fieldset><legend> -A- </legend><input type="text"></fieldset><fieldset><legend> -B- </legend><input type="text"></fieldset><fieldset><legend> -C- </legend><input type="text"></fieldset></form>