Non-horizontal form-group using fieldset renders legend font-size different than label

When using a <fieldset> as the root element of a non-horizontal .form-group (i.e. no .form-row or .row), the <legend> element renders differently than a <label> (specifically the font-size being set to 1.5rem via reboot)

<div class="form-group">
  <label for="input">Label</label>
  <input type="text" class="form-control">
</div>

<fieldset class="form-group">
  <legend>Foobar</legend>
  <div role="group">
    <div class="form-check form-check-inline">
      <input id="check1" class="form-check-input" type="checkbox" name="checks" value="A">
      <label for="check1" class="form-check-label">Radio A</label> 
    </div>
    <div class="form-check form-check-inline">
      <input id="check2" class="form-check-input" type="checkbox" name="checks" value="B">
      <label for="check2" class="form-check-label">Radio B</label> 
    </div>
  </div>
</fieldset>

Renders:

Manually adding a style font-size: inherit; or font-size: 1rem; to the <legend> appears to fix this issue, but it would be preferential not to have to add the additional styling.

Proposed solution (to mimic <label> default) in /scss/_forms.scss before the .col-form-label rule definition:

.form-group legend {
  font-size: inherit;
}

Which renders the following (using the same above HTML):

[mdo]

Oh I agree it should be improved, but overriding like that could cause issues for folks who’ve built around this existing behavior.

Since it is suggested it can be used on fieldsets, the form-group component should probably treat labels and legends the same stylistically.

Eh, just because a particular HTML element can have a class doesn’t mean something nested within that element should change. Again, I agree there should be some control here, just not in the solution offered here thus far.

In all likelihood, we should move from general label { ... } rulesets to .form-label { ... } to help avoid issues like this as we could just apply that class. Perhaps for now v4 should have label, .form-label { ... } or something akin?

[akrawchyk]

@tmorehouse looks like a $legend-font-size variable was added to allow theming the default <legend> element: https://github.com/twbs/bootstrap/commit/855d763ae57fd1dea1c26af1f197d3d84002dc55#diff-d8ee409a461718bfb6240710c8c73382R345

Can this be backported to v4?