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CSS

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JavaScript

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HTML 

              
                - var n_rows = 7, p = [];	

style
	- for(var ridx = 1; ridx < n_rows; ridx++) {
		- var col_lim_inf = n_rows - ridx;
		- var col_lim_sup = n_rows + ridx;
		- var idx_lim_inf = Math.pow(ridx, 2) + 1;
		- var idx_lim_sup = idx_lim_inf + 2*ridx;
		- var a = [[idx_lim_inf], [idx_lim_sup]];
		- var na = a.length;
		- for(var i = 1; i < n_rows - ridx; i++) {
			- for(var j = 0; j < na; j++) {
				- var k = a[j][i - 1] + 2*(i + ridx)
				- a[j].push(k);
			- }
		- }
		- var sel = `.s3gon:nth-child(n + ${idx_lim_inf}):nth-child(-n + ${idx_lim_sup})`
		- if(ridx%2)
			- p.push(sel)
		| #{sel} { --ridx: #{ridx + 1} }
		| .s3gon:nth-child(#{a[0].join('), .s3gon:nth-child(')}) { --cidx: #{col_lim_inf} }
		| .s3gon:nth-child(#{a[1].join('), .s3gon:nth-child(')}) { --cidx: #{col_lim_sup} }
	- }
	| #{p.join(', ')} { --p0: -1 }
.grid(style=`--n-rows: ${n_rows}; --n-cols: ${2*n_rows + 1}; --cidx: ${n_rows}`)
	- for(var ridx = 0; ridx < n_rows; ridx++)
		- for(var cidx = n_rows - ridx; cidx <= n_rows + ridx; cidx++)
			.s3gon
!

CSS 

              
                @import 'compass/css3';

@function getPolyPoints(
		$n: 3 /* number of poly vertices */, 
		$oa: -90deg /* angular offset of 1st poly vertex */, 
		$bw: 2px /* polygon border-width */) {
	
	$ba: 360deg/$n; // base angle corrensponding to 1 poly edge
	$l0: (); // list of points, initially empty
	$l1: (); // list of points, initially empty
	
	@for $i from 0 through $n {
		$ca: $i*$ba + $oa; // angle current point is at wrt x axis
		$x: calc(50%*(1 + #{cos($ca)})); // x coord of current point
		$y: calc(50%*(1 + var(--p)*#{sin($ca)})); // y coord of current point
		$l0: $l0, $x $y; // add current point coords to points list
		$ca: -$i*$ba + $oa; // angle current point is at wrt x axis
		$x: calc(50% + (50% - #{$bw})*#{cos($ca)}); // x coord of current point
		$y: calc(50% + (50% - #{$bw})*var(--p)*#{sin($ca)}); // y coord of current point
		$l1: $l1, $x $y // add current point coords to points list
	}
	
	@return join($l0, $l1, comma)
}

$c: #ff1ead;
$l: 6.5vmin;
$t: 1s;

$n3gon: 3;
$ba3gon: 360deg/$n3gon;
$rc3gon: .5*$l/sin(.5*$ba3gon);
$dc3gon: 2*$rc3gon;
$ri3gon: .5*$l/tan(.5*$ba3gon);
$h3gon: .5*$l*sqrt(3);
$mv: .5*($h3gon - $dc3gon);
$mh: .5*(.5*$l - $dc3gon);

body {
	display: grid;
	place-content: center;
	margin: 0;
	height: 100vh;
	background: $c
}

.grid {
	display: grid;
	padding: -$mv (-$mh);
	//filter: drop-shadow(2px 2px 5px)
}

.s3gon {
	--m: calc(.5*(1 - var(--p0, 1)*var(--p1, 1)));
	--p: calc(2*var(--m) - 1);
	grid-row: var(--ridx);
	grid-column: var(--cidx, var(--n-rows));
	position: relative;
	margin: calc(#{$mv} - (1 - var(--m))*#{$ri3gon}) $mh;
	width: $dc3gon; height: $dc3gon;
	background: currentcolor;
	color: #000;
	clip-path: polygon(getPolyPoints());
	animation: size $t infinite alternate;
	animation-delay: calc(var(--cidx)*#{-$t}/var(--n-cols));
	animation-duration: $t, 2*$t;
	animation-name: size, cinv, fill;
	animation-timing-function: ease-in, steps(1), ease-out;

	&:nth-child(odd) { --p1: -1 }
}

@keyframes size { 95%, 100% { transform: scale(0) } }

@keyframes fill { 95%, 100% { clip-path: polygon(getPolyPoints($bw: 50%)) } }

@keyframes cinv { 50% { color: #fff } }
!

JS 

              
                // no JS here, but check out version #1 of this demo https://codepen.io/thebabydino/pen/KJddbK
!
999px

Console