SOURCE

/*
	This pen cleverly utilizes SVG filters to create a "Morphing Text" effect. Essentially, it layers 2 text elements on top of each other, and blurs them depending on which text element should be more visible. Once the blurring is applied, both texts are fed through a threshold filter together, which produces the "gooey" effect. Check the CSS - Comment the #container rule's filter out to see how the blurring works!
*/

const elts = {
	text1: document.getElementById("text1"),
	text2: document.getElementById("text2")
};

// The strings to morph between. You can change these to anything you want!
const texts = [
	"Why",
	"is",
	"this",
	"so",
	"satisfying",
	"to",
	"watch?"
];

// Controls the speed of morphing.
const morphTime = 1;
const cooldownTime = 0.25;

let textIndex = texts.length - 1;
let time = new Date();
let morph = 0;
let cooldown = cooldownTime;

elts.text1.textContent = texts[textIndex % texts.length];
elts.text2.textContent = texts[(textIndex + 1) % texts.length];

function doMorph() {
	morph -= cooldown;
	cooldown = 0;
	
	let fraction = morph / morphTime;
	
	if (fraction > 1) {
		cooldown = cooldownTime;
		fraction = 1;
	}
	
	setMorph(fraction);
}

// A lot of the magic happens here, this is what applies the blur filter to the text.
function setMorph(fraction) {
	// fraction = Math.cos(fraction * Math.PI) / -2 + .5;
	
	elts.text2.style.filter = `blur(${Math.min(8 / fraction - 8, 100)}px)`;
	elts.text2.style.opacity = `${Math.pow(fraction, 0.4) * 100}%`;
	
	fraction = 1 - fraction;
	elts.text1.style.filter = `blur(${Math.min(8 / fraction - 8, 100)}px)`;
	elts.text1.style.opacity = `${Math.pow(fraction, 0.4) * 100}%`;
	
	elts.text1.textContent = texts[textIndex % texts.length];
	elts.text2.textContent = texts[(textIndex + 1) % texts.length];
}

function doCooldown() {
	morph = 0;
	
	elts.text2.style.filter = "";
	elts.text2.style.opacity = "100%";
	
	elts.text1.style.filter = "";
	elts.text1.style.opacity = "0%";
}

// Animation loop, which is called every frame.
function animate() {
	requestAnimationFrame(animate);
	
	let newTime = new Date();
	let shouldIncrementIndex = cooldown > 0;
	let dt = (newTime - time) / 1000;
	time = newTime;
	
	cooldown -= dt;
	
	if (cooldown <= 0) {
		if (shouldIncrementIndex) {
			textIndex++;
		}
		
		doMorph();
	} else {
		doCooldown();
	}
}

// Start the animation.
animate();

<!-- Explanation in JS tab -->

<!-- The two texts -->
<div id="container">
	<span id="text1"></span>
	<span id="text2"></span>
</div>

<!-- The SVG filter used to create the merging effect -->
<svg id="filters">
	<defs>
		<filter id="threshold">
			<!-- Basically just a threshold effect - pixels with a high enough opacity are set to full opacity, and all other pixels are set to completely transparent. -->
			<feColorMatrix in="SourceGraphic"
					type="matrix"
					values="1 0 0 0 0
									0 1 0 0 0
									0 0 1 0 0
									0 0 0 255 -140" />
		</filter>
	</defs>
</svg>

/* Explanation in JS tab */

/* Cool font from Google Fonts! */
@import url('https://fonts.googleapis.com/css?family=Raleway:900&display=swap');

body {
	margin: 0px;
}

#container {
	/* Center the text in the viewport. */
	position: absolute;
	margin: auto;
	width: 100vw;
	height: 80pt;
	top: 0;
	bottom: 0;
	
	/* This filter is a lot of the magic, try commenting it out to see how the morphing works! */
	filter: url(#threshold) blur(0.6px);
}

/* Your average text styling */
#text1, #text2 {
	position: absolute;
	width: 100%;
	display: inline-block;
	
	font-family: 'Raleway', sans-serif;
	font-size: 80pt;
	
	text-align: center;
	
	user-select: none;
}