Can blocks have other blocks as children, or only elements are allowed?

[andrejesuscm]

Imagine the following component:

<div class="card">
    <div class="heading">...</div>
    <div class="card__body">...</div>
    <div class="card__footer">...</div>
</div>

Is it allowed to have another block heading as children of another block, or should we wrap it in another element:

<div class="card">
    <div class="card_heading">
        <div class="heading">...</div>
    </div>
    <div class="card__body">...</div>
    <div class="card__footer">...</div>
</div>

Thanks for the clarification.