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HTML

              
                <!--
Schau auf Codepalm.de für eine detaillierte Erklärung: https://www.codepalm.de/post/lazy-loading-bilder-nachladen-lassen/

Lazy Loading funktioniert leider nicht auf Codepen. 
Das Skript kannst du auf der Startseite von Codepalm.de Live in Action sehen.
-->

<img class="lazy-load" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAABBJREFUeNpi+P//PwNAgAEACPwC/tuiTRYAAAAASUVORK5CYII=" data-src="https://images.unsplash.com/photo-1539692397242-335cd88cf0a9?ixlib=rb-1.2.1&q=85&fm=jpg&crop=entropy&cs=srgb&ixid=eyJhcHBfaWQiOjE0NTg5fQ" />

<img class="lazy-load" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAABBJREFUeNpi+P//PwNAgAEACPwC/tuiTRYAAAAASUVORK5CYII=" data-src="https://images.unsplash.com/photo-1484662020986-75935d2ebc66?ixlib=rb-1.2.1&q=85&fm=jpg&crop=entropy&cs=srgb&ixid=eyJhcHBfaWQiOjE0NTg5fQ" />

<img class="lazy-load" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAABBJREFUeNpi+P//PwNAgAEACPwC/tuiTRYAAAAASUVORK5CYII=" data-src="https://images.unsplash.com/photo-1519430339248-0f8ecae7170b?ixlib=rb-1.2.1&q=85&fm=jpg&crop=entropy&cs=srgb&ixid=eyJhcHBfaWQiOjE0NTg5fQ" />

<img class="lazy-load" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAGXRFWHRTb2Z0d2FyZQBBZG9iZSBJbWFnZVJlYWR5ccllPAAAABBJREFUeNpi+P//PwNAgAEACPwC/tuiTRYAAAAASUVORK5CYII=" data-src="https://images.unsplash.com/photo-1561192740-c086a25dcaf4?ixlib=rb-1.2.1&q=85&fm=jpg&crop=entropy&cs=srgb&ixid=eyJhcHBfaWQiOjE0NTg5fQ" />
              
            
!

CSS

              
                .fade-in {
	animation-name: fadeIn;
	animation-duration: 1.3s;
	animation-timing-function: cubic-bezier(0, 0, 0.4, 1);
	animation-fill-mode: forwards;
}
@keyframes fadeIn {
  from {
	  opacity: 0;
  }
  to {
	  opacity: 1;
  }
}

/* Nicht relevant für das Skript */
img {
  max-width: 100%;
}

/* See "Broken-Images" (https://codepen.io/Codepalm/pen/rEYWEa) */
img {
    min-height: 50px;
    display: block;
    position: relative;
}
img:before {
    content: " ";
    display: block;
    position: absolute;
    top: 0; 
    left: 0;
    height: -webkit-calc( 100% + 10px );
    height: calc( 100% + 10px );
    width: 100%;
    background-color: rgb(230,230,230);
    border: 2px dotted rgb(200,200,200);
    border-radius: 3px;
}
img:after {
    position: absolute;
    top: 0px;
    bottom: 0px;
    left: 0px;
    right: 0px;
    width: 100%;
    height: -webkit-calc( 100% + 10px );
    height: calc( 100% + 10px );
    padding: 10px 0;
    border-radius: 3px;
    content: "\f127   Fehlerhaftes Bild '" attr(alt) "'";
    display: block;
    font-size: 16px;
    font-style: normal;
    font-family: FontAwesome;
    color: rgb(100,100,100);
    text-align: center;
}

/* Image is in Loading-Process */
img.lazy-load:not([src]):after {
    content: "Bild '" attr(alt) "' wird geladen";
}
              
            
!

JS

              
                const images = document.querySelectorAll( '.lazy-load' );
const config = {
	rootMargin: '50px 0px',
	threshold: 0.01
};

let imageCount = images.length;
let observer;

if( !( 'IntersectionObserver' in window ) ) {
  loadImagesImmediately( images );
}
else {
	observer = new IntersectionObserver( onIntersection, config );
	for (let i = 0; i < images.length; i++) { 
		let image = images[i];
		if( image.classList.contains( 'js-lazy-image--handled' ) )
			continue;
		observer.observe( image );
	}
}

function fetchImage( url ) {
	return new Promise((resolve, reject) => {
		const image = new Image();
		image.src = url;
		image.onload = resolve;
		image.onerror = reject;
	});
}

function preloadImage( image ) {
	const src = image.dataset.src;
	if( !src )
		return;
	return fetchImage( src ).then(() => { applyImage( image, src ); }, () => { applyImage( image, '#' ); });
}

function loadImagesImmediately( images ) {
	for( let i=0; i < images.length; i++ ) { 
		let image = images[i];
		preloadImage( image );
	}
}

function disconnect() {
	if( !observer )
		return;
	observer.disconnect();
}

function onIntersection( entries ) {
	if( imageCount === 0 ) {
		disconnect();
		return;
	}
	
	for( let i=0; i < entries.length; i++ ) {
		let entry = entries[i];
		if( entry.intersectionRatio > 0 ) {
			imageCount--;
			observer.unobserve( entry.target );
			preloadImage( entry.target );
		}
	}
}

function applyImage( img, src ) {
	if( src != "#" ) {
		img.classList.add( 'js-lazy-image--handled' );
		img.src = src;
		img.classList.add( 'fade-in' );
	}
	else {
		img.src = src;
		img.classList.add( 'js-lazy-image--error' );
	}
}
              
            
!
999px

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