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HTML 

              
                <h1>Creating new Selectors with <code>:has()</code>:<br /> The <code>:nth-in-island-of-class</code>, and <code>:nth-last-in-island-of-class</code> selector generator</h1>

<h2>What?</h2>
<p>This page is a demo for <a href="https://brm.us/css-islands" target="_top">https://brm.us/css-islands</a>. Please read the post to get the full details and explanation.</p>

<h2>Selector Generator</h2>

<p>I want to target the <select name="n" id="n"><option value="1">1st</option><option value="2">2nd</option><option value="3" selected>3rd</option><option value="4">4th</option><option value="5">5th</option><option value="6">6th</option><option value="7">7th</option><option value="8">8th</option><option value="9">9th</option><option value="10">10th</option></select> <select name="type" id="type"><option value="nth">:nth-in-island-of-class</option><option value="nth-last">:nth-last-in-island-of-class</option></select> with selector <input type="text" name="selector" id="selector" value=".special"/></p>

<pre><code id="output">[loading …]</code></pre>

<h3>Demo</h3>
<div class="no-support" data-support="css-has-basic"><p>🚨 Your browser does not support CSS <code>:has()</code>, so this demo will not work correctly.</p></div>
<div class="no-support" data-support="css-has-relative"><p>🚨 Your browser does not support relative selectors in CSS <code>:has()</code>, so this demo will not work correctly.</p></div>

<p><em>TIP: You can click each paragraph to toggle the <code>.special</code> class</em></p>

<div class="demo">
	<p>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eaque in, enim rem recusandae mollitia error, natus ab atque dignissimos culpa veniam exercitationem sequi voluptates, commodi corrupti ullam sit. Debitis, a!</p>
	<p class="special">Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eaque in, enim rem recusandae mollitia error, natus ab atque dignissimos culpa veniam exercitationem sequi voluptates, commodi corrupti ullam sit. Debitis, a!</p>
	<p class="special">Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eaque in, enim rem recusandae mollitia error, natus ab atque dignissimos culpa veniam exercitationem sequi voluptates, commodi corrupti ullam sit. Debitis, a!</p>
	<p class="special">Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eaque in, enim rem recusandae mollitia error, natus ab atque dignissimos culpa veniam exercitationem sequi voluptates, commodi corrupti ullam sit. Debitis, a!</p>
	<p class="special">Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eaque in, enim rem recusandae mollitia error, natus ab atque dignissimos culpa veniam exercitationem sequi voluptates, commodi corrupti ullam sit. Debitis, a!</p>
	<p>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eaque in, enim rem recusandae mollitia error, natus ab atque dignissimos culpa veniam exercitationem sequi voluptates, commodi corrupti ullam sit. Debitis, a!</p>
	<p>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eaque in, enim rem recusandae mollitia error, natus ab atque dignissimos culpa veniam exercitationem sequi voluptates, commodi corrupti ullam sit. Debitis, a!</p>
	<p>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eaque in, enim rem recusandae mollitia error, natus ab atque dignissimos culpa veniam exercitationem sequi voluptates, commodi corrupti ullam sit. Debitis, a!</p>
	<p class="special">Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eaque in, enim rem recusandae mollitia error, natus ab atque dignissimos culpa veniam exercitationem sequi voluptates, commodi corrupti ullam sit. Debitis, a!</p>
	<p class="special">Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eaque in, enim rem recusandae mollitia error, natus ab atque dignissimos culpa veniam exercitationem sequi voluptates, commodi corrupti ullam sit. Debitis, a!</p>
	<p>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eaque in, enim rem recusandae mollitia error, natus ab atque dignissimos culpa veniam exercitationem sequi voluptates, commodi corrupti ullam sit. Debitis, a!</p>
	<p class="special">Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eaque in, enim rem recusandae mollitia error, natus ab atque dignissimos culpa veniam exercitationem sequi voluptates, commodi corrupti ullam sit. Debitis, a!</p>
	<p>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eaque in, enim rem recusandae mollitia error, natus ab atque dignissimos culpa veniam exercitationem sequi voluptates, commodi corrupti ullam sit. Debitis, a!</p>
	<p class="special">Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eaque in, enim rem recusandae mollitia error, natus ab atque dignissimos culpa veniam exercitationem sequi voluptates, commodi corrupti ullam sit. Debitis, a!</p>
	<p>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eaque in, enim rem recusandae mollitia error, natus ab atque dignissimos culpa veniam exercitationem sequi voluptates, commodi corrupti ullam sit. Debitis, a!</p>
	<p class="special">Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eaque in, enim rem recusandae mollitia error, natus ab atque dignissimos culpa veniam exercitationem sequi voluptates, commodi corrupti ullam sit. Debitis, a!</p>
	<p class="special">Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eaque in, enim rem recusandae mollitia error, natus ab atque dignissimos culpa veniam exercitationem sequi voluptates, commodi corrupti ullam sit. Debitis, a!</p>
	<p class="special">Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eaque in, enim rem recusandae mollitia error, natus ab atque dignissimos culpa veniam exercitationem sequi voluptates, commodi corrupti ullam sit. Debitis, a!</p>
	<p>Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eaque in, enim rem recusandae mollitia error, natus ab atque dignissimos culpa veniam exercitationem sequi voluptates, commodi corrupti ullam sit. Debitis, a!</p>
	<p class="special">Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eaque in, enim rem recusandae mollitia error, natus ab atque dignissimos culpa veniam exercitationem sequi voluptates, commodi corrupti ullam sit. Debitis, a!</p>
	<p class="special">Lorem ipsum dolor sit, amet consectetur adipisicing elit. Eaque in, enim rem recusandae mollitia error, natus ab atque dignissimos culpa veniam exercitationem sequi voluptates, commodi corrupti ullam sit. Debitis, a!</p>
</div>
!

CSS 

              
                :where(.demo > *) {
	margin: 0;
	padding: 0.5em 1em;
	border: 1px solid transparent; /* We set a transparent border on all sides to prevent a layout jump when actually painting the border */
	cursor: pointer;
	transition: all 0.5s ease-in-out;
}

/* Basic .special styling */
.special {
	background: rgb(0 0 0 / 0.1);
	border-left: 1px solid currentcolor;
}

/* Non-demo styles below */
@layer base {
	@layer layout {
		html {
			max-width: 84ch;
			padding: 3rem 2rem;
			margin: auto;
			line-height: 1.5;
			font-size: 1.25rem;
		}
		body {
			margin: 0;
			padding: 0;
		}
		a,
		a:visited {
			color: blue;
		}
		
		h2 {
			margin-top: 2em;
		}
		
		h1 {
			display: none;
		}
	}
	
	@layer code {
		pre {
			border: 1px solid #dedede;
			padding: 1em;
			background: #f7f7f7;
			font-family: "Courier 10 Pitch", Courier, monospace;
			overflow-x: auto;
			border-left: 0.4em solid cornflowerblue;
			tab-size: 4;
		}
		
		code:not(pre code) {
			background: #f7f7f7;
			border: 1px solid rgb(0 0 0 / 0.2);
		}
	}

	@layer support {
		.no-support,
		.has-support {
			margin: 1em 0;
			padding: 1em;
			border: 1px solid #ccc;
		}

		.no-support {
			background-color: #ff00002b;
			display: block;
		}
		.has-support {
			background-color: #00ff002b;
			display: none;
		}
		:is(.has-support, .no-support) > :first-child {
			margin-top: 0;
		}
		:is(.has-support, .no-support) > :last-child {
			margin-bottom: 0;
		}

		@supports(selector(:has(*))) {
			.no-support[data-support="css-has-basic"] {
				display: none;
			}
			.has-support[data-support="css-has-basic"] {
				display: block;
			}
		}

		@supports(selector(:has(+ *))) {
			.no-support[data-support="css-has-relative"] {
				display: none;
			}
			.has-support[data-support="css-has-relative"] {
				display: block;
			}
		}

		@supports(selector(:nth-child(1 of .foo))) {
			.no-support[data-support="css-nth-child-of-s"] {
				display: none;
			}
			.has-support[data-support="css-nth-child-of-s"] {
				display: block;
			}
		}
	}
}

@layer reset {
	
}
!

JS 

              
                document.querySelectorAll('.demo p').forEach($p => {
	$p.addEventListener('click', (e) => {
		e.target.classList.toggle('special');
	});
});

const createDynamicStyleElement = (id) => {
    var style = document.createElement("style");
    style.type = "text/css";
    style.id = id;
    return style;
}

const generateSelector = (n, type, selector) => {
	
	n = parseInt(n);
	if (n === 0) {
		throw new Error('Invalid value for n');
	}
	
	let suffix = '';
	
	switch (type) {
		case `nth`:
			return `${selector}:not(${selector} + ${selector}) ${`+ ${selector} `.repeat(n-1).trim()}`;
			break;
			
		case 'nth-last':
			
			if (n > 1) {
				suffix = `:has(${`+ ${selector} `.repeat(n-1).trim()})`;
			}		
			
			return `.special:not(:has(${`+ ${selector} `.repeat(n).trim()}))${suffix}`
			
			break;
			
		default:
			throw new Error('Invalid type');
	}
}

const updateStyles = (selector) => {
	let $style = document.querySelector('#dynamic-styles');
	if (!$style) {
		$style = createDynamicStyleElement('dynamic-styles');
		document.head.appendChild($style);
	}
	
	const styleSheet = Array.from(document.styleSheets).filter(s => s.ownerNode == $style)[0];
	
	if (styleSheet.cssRules.length) {
		styleSheet.deleteRule(0);
	}
	
	styleSheet.insertRule(`${selector} { color: red; }`, 0);
}

const updateOutput = (selector) => {
	document.querySelector('#output').innerText = `${selector} {
    …
}`;
}

const update = () => {
	const selector = generateSelector(
		document.getElementById('n').value,
		document.getElementById('type').value,
		document.getElementById('selector').value,
	);
	updateStyles(selector);
	updateOutput(selector);
}

document.querySelectorAll('select, input').forEach($el => {
	$el.addEventListener('change', (e) => {
		update();
	});
});

update();
!
999px

Console