html - <input> has mysterious bottom padding

Question

I am trying to do some quite precise styling on some form elements, and this issue is causing me a lot of grief.

If I try to remove padding, margin, border and outline from an <input> (with display: block) so that the size of the field is purely determined by the text, the input field ends up having a few pixels extra padding than any other block level element styled exactly the same way. Here's an example: http://jsfiddle.net/nottrobin/b9zfa/

<input class="normalised testSubject" value="hello" />
<div class="normalised testSubject">hello</div>

Rendering:

In that example, the <div> gets a computed height of 16px while the <input> gets a computed height of 19px.

I get the same behaviour in Chrome 16, Firefox 9 and Opera 11 so it's clearly rendering engine independent.

I can fix the issue by manually adding a height, but I don't want to do that because I want the design to remain responsive.

Can anyone help me understand what's going on here, and how I can reliably make sure that the <input> will be the same height as any block level element that follows it?

Answer 1

The <input> has a minimum line-height based on font size. Setting both elements to a larger line-height value works, as does removing line-height altogether. But that still doesn't allow you to have smaller heights than the minimum. The fix for that is using the first-line pseudo-element and setting it to display: inline-block;.

Demo: http://jsfiddle.net/ThinkingStiff/B7cmQ/

CSS:

.normalised:first-line {
    display: inline-block;    
}

But this doesn't explain why the <input> is acting differently than the <div>. Even -webkit-appearance: none; didn't fix it. It would seem there is some invisible voodoo on inputs that treats its contents as inline. inline elements have minimun line-height based on font size, which is the behavior we're seeing here. That's why first-line fixes it. It seems to be styling the "child" element of the <input>.

Here's a demo that shows the minimum line-height on inline elements. The <div> element honors line-height: 7px;. The <span>, even though its computed value is showing 7px;, is not honoring it visually.

Demo: http://jsfiddle.net/ThinkingStiff/zhReb/

Output:

HTML:

<div id="container"> 
    <div id="div-large">div <br />large</div> 
</div> 
<div id="container"> 
    <div id="div-medium">div <br />med</div> 
</div> 
<div id="container"> 
    <div id="div-small">div <br />small</div> 
</div> 
<div id="container"> 
    <span id="span-large">span <br />large</span> 
</div> 
<div id="container"> 
    <span id="span-medium">span <br />med</span> 
</div> 
<div id="container"> 
    <span id="span-small">span <br />small</span> 
</div> 

CSS:

#container { 
    background-color: lightblue;   
    display: inline-block; 
    height: 200px; 
    vertical-align: top; 
}

#div-large { 
    line-height: 50px; 
} 

#div-medium { 
    line-height: 20px; 
} 

#div-small { 
    line-height: 7px; 
}

#span-large { 
    line-height: 50px; 
    vertical-align: top; 
} 

#span-medium {
    line-height: 20px; 
    vertical-align: top; 
} 

#span-small {
    line-height: 7px;
    vertical-align: top;
}