BEM - Block, Element, Modifier
1. Block
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Eg: menu
<ul class="menu"></ul>
2. Elements
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Eg: menu__list
<ul class="menu">
<li class="menu__list"></li>
</ul>
3. Modifiers
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Eg: menu--active
<ul class="menu">
<li class="menu__list"></li>
<li class="menu__list menu--active"></li>
</ul>