Somehow if I put items-center for the parent, a child element changes its width.
I use tailwind CSS.
<link href="https://cdnjs.cloudflare.com/ajax/libs/tailwindcss/2.2.19/tailwind.min.css" rel="stylesheet"/>
<div class="flex flex-col items-center">
<div class="mx-5">
<div class="flex max-w-xl mx-auto border-solid border border-slate-300/50 h-10 ">
<input type="text" class="bg-inherit outline-none">
</div>
</div>
</div>