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HTML

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Insecure Resource

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CSS

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CSS preprocessors help make authoring CSS easier. All of them offer things like variables and mixins to provide convenient abstractions.

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CSS Base

About CSS Base

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Vendor Prefixing

About Vendor Prefixing

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JavaScript

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Babel includes JSX processing.

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HTML 

              
                <div class="container __loading">

<ul>
		<li>コーディング中は、あえてファイルサイズが重い画像を使うのがおすすめです。<br>（読み込み時間が長くなってアニメーションの挙動を確認しやすいので）</li>
		<li>ページをリロードする時は、ハード再読み込み（Command＋Shift＋R）しましょう。<br>（画像のキャッシュが残っていると読み込み時間が短くなってしまうので）</li>
</ul>

<!-- ########################################## -->
<!-- グレーの透明度が点滅するアニメーション -->
<!-- ########################################## -->
<div class="wrap-fade-toggle">

	<div class="card">
		<div class="card__img anima-fade-toggle">
			<img src="https://cdn.spacetelescope.org/archives/images/publicationjpg/heic1901a.jpg" alt="" width="200" height="150">
		</div>
		<div class="card__ttl anima-fade-toggle">heic1901a</div>
	</div>

	<div class="card">
		<div class="card__img anima-fade-toggle">
			<img src="https://cdn.spacetelescope.org/archives/images/publicationjpg/heic1808a.jpg" alt="" width="200" height="150">
		</div>
		<div class="card__ttl anima-fade-toggle">heic1808a</div>
	</div>

	<div class="card">
		<div class="card__img anima-fade-toggle">
			<img src="https://cdn.spacetelescope.org/archives/images/large/heic0821b.jpg" alt="" width="200" height="150">
		</div>
		<div class="card__ttl anima-fade-toggle">heic0821b</div>
	</div>

	<div class="card">
		<div class="card__img anima-fade-toggle">
			<img src="https://cdn.spacetelescope.org/archives/images/large/potw1441a.jpg" alt="" width="200" height="150">
		</div>
		<div class="card__ttl anima-fade-toggle">potw1441a</div>
	</div>

	<div class="card">
		<div class="card__img anima-fade-toggle">
			<img src="https://cdn.spacetelescope.org/archives/images/large/heic0715a.jpg" alt="" width="200" height="150">
		</div>
		<div class="card__ttl anima-fade-toggle">heic0715a</div>
	</div>

	<div class="card">
		<div class="card__img anima-fade-toggle">
			<img src="https://cdn.spacetelescope.org/archives/images/large/potw2114a.jpg" alt="" width="200" height="150">
		</div>
		<div class="card__ttl anima-fade-toggle">potw2114a</div>
	</div>

</div>

<hr>

<!-- ########################################## -->
<!-- グラデーションが移動するアニメーション -->
<!-- ########################################## -->
<div class="wrap-gradient">

	<div class="card">
		<div class="card__img anima-gradient">
			<img src="https://cdn.spacetelescope.org/archives/images/publicationjpg/heic1901a.jpg" alt="" width="200" height="150">
		</div>
		<div class="card__ttl anima-gradient">heic1901a</div>
	</div>

	<div class="card">
		<div class="card__img anima-gradient">
			<img src="https://cdn.spacetelescope.org/archives/images/publicationjpg/heic1808a.jpg" alt="" width="200" height="150">
		</div>
		<div class="card__ttl anima-gradient">heic1808a</div>
	</div>

	<div class="card">
		<div class="card__img anima-gradient">
			<img src="https://cdn.spacetelescope.org/archives/images/large/heic0821b.jpg" alt="" width="200" height="150">
		</div>
		<div class="card__ttl anima-gradient">heic0821b</div>
	</div>

	<div class="card">
		<div class="card__img anima-gradient">
			<img src="https://cdn.spacetelescope.org/archives/images/large/potw1441a.jpg" alt="" width="200" height="150">
		</div>
		<div class="card__ttl anima-gradient">potw1441a</div>
	</div>

	<div class="card">
		<div class="card__img anima-gradient">
			<img src="https://cdn.spacetelescope.org/archives/images/large/heic0715a.jpg" alt="" width="200" height="150">
		</div>
		<div class="card__ttl anima-gradient">heic0715a</div>
	</div>

	<div class="card">
		<div class="card__img anima-gradient">
			<img src="https://cdn.spacetelescope.org/archives/images/large/potw2114a.jpg" alt="" width="200" height="150">
		</div>
		<div class="card__ttl anima-gradient">potw2114a</div>
	</div>

</div>
	
</div>
!

CSS 

              
                /* ############################################## */
/* 共通（アニメーションとは直接関係のないスタイル） */
/* ############################################## */

body{
	padding: 20px;
}

.wrap-fade-toggle,
.wrap-gradient{
	display: flex;
	flex-wrap: wrap;
}

.card{
	width: 200px;
	margin: 4%;
}
.card__img{
	position: relative; /* 両方のアニメーションで必要 */
	overflow: hidden; /* グラデーションが移動するアニメーションで必要 */
}
.card__ttl{
	display: inline-block;
	font-weight: bold;
	margin-top: 8px;
	position: relative; /* 両方のアニメーションで必要 */
	overflow: hidden; /* グラデーションが移動するアニメーションで必要 */
}



/* ############################################## */
/* グレーの透明度が点滅するアニメーション */
/* ############################################## */

/* 重ねるグレーのベース */
.anima-fade-toggle::before {
	content: "";
	background-color: #eee;
	position: absolute;
	top: 0;
	left: 0;
	width: 100%;
	height: 100%;
	transition: opacity .4s, visibility .4s;
}

/* 読み込み中 */
.container.__loading .anima-fade-toggle::before {
	animation: anima_fade_toggle 1s infinite;
	animation-direction: alternate;
}
@keyframes anima_fade_toggle {
	0% {
		background-color: #eee;
	}
	100% {
		background-color: #bbb;
	}
}

/* 読み込み完了後 */
.container.__loaded .anima-fade-toggle::before {
	opacity: 0;
	visibility: hidden;
}



/* ############################################## */
/* グラデーションが移動するアニメーション */
/* ############################################## */

/* 重ねるグレーのベース */
.anima-gradient::before {
	content: "";
	background: linear-gradient(90deg, #f1f1f1 0%, #ccc 50%, #f1f1f1 100%);
	position: absolute;
	top: 0;
	left: 0;
	width: 300%;
	height: 100%;
	z-index: 1;
	transition: opacity .4s, visibility .4s;
}
.anima-gradient::after {
	content: "";
	background-color: #f1f1f1;
	position: absolute;
	top: 0;
	left: 0;
	width: 100%;
	height: 100%;
	transition: opacity .4s, visibility .4s;
}

/* 読み込み中 */
.container.__loading .anima-gradient::before {
	animation: anima_gradient 2.2s ease-in-out infinite;
	opacity: 0.9;
}
@keyframes anima_gradient {
	0% {
		left: -300%;
	}
	100% {
		left: 300%;
	}
}

/* 読み込み完了後 */
.container.__loaded .anima-gradient::before,
.container.__loaded .anima-gradient::after {
	opacity: 0;
	visibility: hidden;
}
!

JS 

              
                // DOMや画像など全てを読み込み後、.containerのクラスを切替え
window.addEventListener('load', function(_ev) {
	document.querySelector('.container').classList.remove('__loading');
	document.querySelector('.container').classList.add('__loaded');
});
!
999px

Console