<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<style>/* dummy placeholder */</style>
<style>
#pseudo1:is(blockquote) {}
</style>
<style>
#pseudo2:is(div) {}
</style>
<style>
:is(#pseudo3):not([hidden]) {}
</style>
<style>
:is(#pseudo4):not(blockquote) {}
</style>
<style>
:where(nonexist, #pseudo5) {}
</style>
</head>
<body>
<blockquote id="pseudo1"></blockquote>
<blockquote id="pseudo2"></blockquote>
<blockquote id="pseudo3"></blockquote>
<blockquote id="pseudo4"></blockquote>
<blockquote id="pseudo5