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Demo from an article on my personal website about the BEM Methodology:

Give this CodePen a ❤️ if you liked it and check out the article :)

<form class="default-form">
	<header class="default-form__form-title">
		<h1 class="default-form__form-title--main-title">Sign In</h1>
		<p class="default-form__form-title--subtitle">get an account, it's great!</p>
	<div class="default-form__form-main-area">
		<input class="default-form__input-box default-form__input-box--username" type="text" placeholder="Enter Username">
		<input class="default-form__input-box default-form__input-box--password" type="password" placeholder="Enter Password">
		<input class="default-form__submit-button" type="button" value="Sign In!">



Things to take note of:

 - The Block, Element, Modifier (block__element--modifier) structure of the classnames
 - The single-class CSS specificity (selectors are just the class, nothing else apart from pseudo-class if relevant) with just the BEM class
 - The multiple classes for modifiers &mdash; some elements have two classes, block__element, and then block__element--modifier
 - Elements are not nested in BEM; nested element classes do not include the parent element, just the block


* {
	margin: 0;
	padding: 0;
	font-family: -apple-system, BlinkMacSystemFont, 'Segoe UI', Roboto, Oxygen, Ubuntu, Cantarell, 'Open Sans', 'Helvetica Neue', sans-serif;
	color: #222222;

.default-form {
	display: block;
	position: absolute;
	top: 40%;
	left: 50%;
	transform: translate(-50%, -50%);
	width: 22.5rem;
	padding: 2.5rem;
	box-shadow: 0 0.375rem 1rem rgba(0, 0, 0, .2);
	border-radius: 3px;

.default-form__form-title {
	display: block;
	margin-bottom: 2rem;

.default-form__form-title--main-title {
	font-size: 2.5rem;
	text-align: center;

.default-form__form-title--subtitle {
	font-size: 1.125rem;
	text-align: center;
	opacity: 0.75;
	margin-top: .5rem;

.default-form__input-box, .default-form__submit-button {
	font-size: 1.125rem;
	padding: .5rem .75rem;
	border: 1px solid #ccc;
	border-radius: 3px;
	display: block;
	margin: 0 auto;
	width: 15rem;
  box-sizing: border-box;

.default-form__input-box::placeholder {
	opacity: .65;

.default-form__input-box:not(:last-child), .default-form__submit-button:not(:last-child) {
	margin-bottom: .75rem;

.default-form__submit-button {
	color: #444;
	background-color: #f0f0f0;