I've come across a bug in the Mac version of Safari where on a number input field I have a vertical line appearing in the box. I've checked it on an iphone as well as Windows based browsers and it isn't present on anything else. Edit: Turnip has pointed out that it happens in Chrome on Windows as well. I've checked it this morning and it happens when you click inside the input box.
The code I'm using to generate the html is:
<div class="quantity">
<input class="minus" type="button" value="-">
<input type="number" step="1" min="1" max="" name="quantity" value="1" title="Qty" class="input-text qty text no-spinners" size="4" pattern="[0-9]*" inputmode="numeric">
<input class="plus" type="button" value="+">
</div>
The link to an example of the page in question is link
Edit: updated the code block to reflect what's rendered client side rather than the server side code.