Fieldset fillet CSS3 compatibility issues

how does fieldset round the border (IE11 is still right-angled Google is rounded)
how to move to 123789 turn red (unless + .red) + li:nth-child (3) unexpectedly not work?

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>fieldset</title>
<style>
fieldset{border-radius:12px;border:2px solid red;width:50%;margin:auto;}
li:nth-child(1):hover +li:nth-child(3){color:red;}
</style>
</head>
<body>
<fieldset><legend>123</legend>456</fieldset>
<ul><li>123</li><li>456</li><li class="red">789</li></ul>
</body>
</html>
Mar.19,2021

it should not be possible to change the style of item 3 of the list while hovering over item 1 of the list, because: hover is a pseudo-class, and it should only work on the element to which it belongs, that is, it can only work on item 1 of the list and the child elements in item 1 of the list.

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