revision:
It can be used on the <label> and <output> elements.
When used together with the <label> element, the "for attribute" specifies which form element a label is bound to, i.e. to specify an "identifier" to the label element.
When used together with the <output> element, the "for attribute" specifies the relationship between the result of the calculation and the elements used in the calculation.
<label for="element_id">
<output for="element_id">
label: element_id: the id of the element the label is bound to.
output: element_id: specifies the relationship between the result of the calculation and the elements used in the calculation.
Click on one of the text labels to toggle the related radio button:
<p class="spec">Click on one of the text labels to toggle the related radio button:</p>
<form style="margin-left:5vw;" action="action_page.php">
<input type="radio" id="html" name="fav_language" value="HTML">
<label for="html">HTML</label><br>
<input type="radio" id="css" name="fav_language" value="CSS">
<label for="css">CSS</label><br>
<input type="radio" id="javascript" name="fav_language" value="JavaScript">
<label for="javascript">JavaScript</labsel><br><br>
<input type="submit" value="Submit">
</form>
<form style="margin-left:5vw;" oninput="x.value=parseInt(a.value)+parseInt(b.value)">
<input type="range" id="a" value="50">
+<input type="number" id="b" value="25">
=<output name="x" for="a b"></output>
</form>