div里再套一个div
<div style="margin:auto;text-align:center;">
<div>
<button class="btn btn-lg" type="button">居中</button>
</div>
</div>
div里再套一个div
<div style="margin:auto;text-align:center;">
<div>
<button class="btn btn-lg" type="button">居中</button>
</div>
</div>