Example of use BEM methodology in Magento 2

bemed-clean.phtml

<form class="form">
    <h2 class="form__header"></h2>
    <p class="form__description"></p>
    <div class="form__field field">
        <label class="field__label"></label>
        <input class="field__input">
    </div>
    <div class="form__field field">
        <label class="field__label"></label>
        <input class="field__input">
    </div>
    <div class="form__field field">
        <label class="field__label"></label>
        <input class="field__input">
    </div>
    <div class="form__field field">
        <label class="field__label"></label>
        <textarea class="field__input field__input--textarea"></textarea>
    </div>

    <div class="form__actions actions-toolbar">
        <div class="actions-toolbar__primary">
            <input class="form__hidden">
            <button class="actions-toolbar__button button"></button>
        </div>
    </div>
</form>

bemed.phtml

<?php // https://github.com/magento/magento2/blob/develop/app/code/Magento/Contact/view/frontend/templates/form.phtml ?>

<?php
    $_helper = $this->helper('Magento\Contact\Helper\Data');
?>

<form class="form"
      id="contact-form"
      action="<?= $block->escapeUrl($block->getFormAction()); ?>"
      method="post"
      data-hasrequired="<?= $block->escapeHtmlAttr(__('* Required Fields')) ?>"
      data-mage-init='{ "validation":{} }'
>
    <h2 class="form__header">
        <?= $block->escapeHtml(__('Write Us')) ?>
    </h2>
    <p class="form__description">
        <?= $block->escapeHtml(__('Jot us a note and we’ll get back to you as quickly as possible.')) ?>
    </p>
    <div class="form__field field">
        <label class="field__label" for="name">
            <?= $block->escapeHtml(__('Name')) ?>
        </label>
        <input class="field__input"
               name="name"
               id="name"
               title="<?= $block->escapeHtmlAttr(__('Name')) ?>"
               value="<?= $block->escapeHtmlAttr($_helper->getPostValue('name') ?: $_helper->getUserName()) ?>"
               type="text"
               data-validate="{ required:true }"
        >
    </div>
    <div class="form__field field">
        <label class="field__label" for="email">
            <?= $block->escapeHtml(__('Email')) ?>
        </label>
        <input class="field__input"
               id="email"
               type="email"
               name="email"
               title="<?= $block->escapeHtmlAttr(__('Email')) ?>"
               value="<?= $block->escapeHtmlAttr($_helper->getPostValue('email') ?: $_helper->getUserEmail()) ?>"
               data-validate="{ required:true, 'validate-email':true }"
        >
    </div>
    <div class="form__field field">
        <label class="field__label" for="telephone">
            <?= $block->escapeHtml(__('Phone Number')) ?>
        </label>
        <input class="field__input"
               name="telephone" 
               id="telephone"
               title="<?= $block->escapeHtmlAttr(__('Phone Number')) ?>"
               value="<?= $block->escapeHtmlAttr($_helper->getPostValue('telephone')) ?>"
               type="text"
        >
    </div>
    <div class="form__field field">
        <label class="field__label" for="comment">
            <?= $block->escapeHtml(__('What’s on your mind?')) ?>
        </label>
        <textarea class="field__input field__input--textarea"
                  id="comment"
                  name="comment"
                  title="<?= $block->escapeHtmlAttr(__('What’s on your mind?')) ?>"
                  cols="5"
                  rows="3"
                  data-validate="{ required:true }"
        ><?= $block->escapeHtml($_helper->getPostValue('comment')) ?></textarea>
    </div>
    <?= $block->getChildHtml('form.additional.info'); ?>

    <div class="form__actions actions-toolbar">
        <div class="actions-toolbar__primary">
            <input class="form__hidden"
                   type="hidden"
                   name="hideit"
                   value=""
            >
            <button class="actions-toolbar__button button"
                    title="<?= $block->escapeHtmlAttr(__('Submit')) ?>"
                    type="submit"
            >
                <?= $block->escapeHtml(__('Submit')) ?>
            </button>
        </div>
    </div>
</form>

source-clean.phtml

<form class="form contact">
    <fieldset class="fieldset">
        <legend class="legend">
            <span></span>
        </legend>
        <br />
        <div class="field note no-label"></div>
        <div class="field name required">
            <label class="label">
                <span></span>
            </label>
            <div class="control">
                <input class="input-text"/>
            </div>
        </div>
        <div class="field email required">
            <label class="label">
                <span></span>
            </label>
            <div class="control">
                <input class="input-text"/>
            </div>
        </div>
        <div class="field telephone">
            <label class="label">
                <span></span>
            </label>
            <div class="control">
                <input class="input-text" />
            </div>
        </div>
        <div class="field comment required">
            <label class="label" for="comment">
                <span></span>
            </label>
            <div class="control">
                <textarea class="input-text"></textarea>
            </div>
        </div>
    </fieldset>
    <div class="actions-toolbar">
        <div class="primary">
            <input />
            <button class="action submit primary">
                <span></span>
            </button>
        </div>
    </div>
</form>

source.phtml

<?php // https://github.com/magento/magento2/blob/develop/app/code/Magento/Contact/view/frontend/templates/form.phtml ?>

<form class="form contact"
      action="<?php echo $block->escapeUrl($block->getFormAction()); ?>"
      id="contact-form"
      method="post"
      data-hasrequired="<?php echo $block->escapeHtmlAttr(__('* Required Fields')) ?>"
      data-mage-init='{"validation":{}}'>
    <fieldset class="fieldset">
        <legend class="legend"><span><?php echo $block->escapeHtml(__('Write Us')) ?></span></legend><br />
        <div class="field note no-label"><?php echo $block->escapeHtml(__('Jot us a note and we’ll get back to you as quickly as possible.')) ?></div>
        <div class="field name required">
            <label class="label" for="name"><span><?php echo $block->escapeHtml(__('Name')) ?></span></label>
            <div class="control">
                <input name="name" id="name" title="<?php echo $block->escapeHtmlAttr(__('Name')) ?>" value="<?php echo $block->escapeHtmlAttr($this->helper('Magento\Contact\Helper\Data')->getPostValue('name') ?: $this->helper('Magento\Contact\Helper\Data')->getUserName()) ?>" class="input-text" type="text" data-validate="{required:true}"/>
            </div>
        </div>
        <div class="field email required">
            <label class="label" for="email"><span><?php echo $block->escapeHtml(__('Email')) ?></span></label>
            <div class="control">
                <input name="email" id="email" title="<?php echo $block->escapeHtmlAttr(__('Email')) ?>" value="<?php echo $block->escapeHtmlAttr($this->helper('Magento\Contact\Helper\Data')->getPostValue('email') ?: $this->helper('Magento\Contact\Helper\Data')->getUserEmail()) ?>" class="input-text" type="email" data-validate="{required:true, 'validate-email':true}"/>
            </div>
        </div>
        <div class="field telephone">
            <label class="label" for="telephone"><span><?php echo $block->escapeHtml(__('Phone Number')) ?></span></label>
            <div class="control">
                <input name="telephone" id="telephone" title="<?php echo $block->escapeHtmlAttr(__('Phone Number')) ?>" value="<?php echo $block->escapeHtmlAttr($this->helper('Magento\Contact\Helper\Data')->getPostValue('telephone')) ?>" class="input-text" type="text" />
            </div>
        </div>
        <div class="field comment required">
            <label class="label" for="comment"><span><?php echo $block->escapeHtml(__('What’s on your mind?')) ?></span></label>
            <div class="control">
                <textarea name="comment" id="comment" title="<?php echo $block->escapeHtmlAttr(__('What’s on your mind?')) ?>" class="input-text" cols="5" rows="3" data-validate="{required:true}"><?php echo $block->escapeHtml($this->helper('Magento\Contact\Helper\Data')->getPostValue('comment')) ?></textarea>
            </div>
        </div>
        <?php echo $block->getChildHtml('form.additional.info'); ?>
    </fieldset>
    <div class="actions-toolbar">
        <div class="primary">
            <input type="hidden" name="hideit" id="hideit" value="" />
            <button type="submit" title="<?php echo $block->escapeHtmlAttr(__('Submit')) ?>" class="action submit primary">
                <span><?php echo $block->escapeHtml(__('Submit')) ?></span>
            </button>
        </div>
    </div>
</form>