Why does container div insist on being slightly larger than IMG or SVG content?

I'm trying to produce yet another lightbox as much needed HTML/CSS/Javascript practice, but I've encountered a styling issue that looks trivial (and probably is!) but I just can't solve it.

I have a div that contains an img. No matter what I try (border, margin, padding, auto height etc.) I just can't make the div shrink to match the image dimensions. I've reduced the problem to this:

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" "http://www.w3.org/TR/html4/strict.dtd">
        <meta http-equiv="Content-Type" content="text/html;charset=utf-8" >
        <title>Layout experiments</title>

        <style type="text/css">
            #lightbox {
                margin: 0;
                padding: 0;
                position    : fixed;
                left        : 50%;
                margin-left : -320px;
                top         : 100px;
                border-radius: 22px;
                background  : #e0e0f0;
                color       : #102020;

            #lightbox img {
                border-radius: 15px;
            .imagebg {
                margin      : 7px;
                background  : black;
                border-radius: 15px;
                height      : 100%;



        <div id="lightbox">
            <div class="imagebg">
                <img src="picture.jpg">

'picture.jpg' is 640x400, but the container div wants to be 640x404, the difference showing itself as a black strip below the image. The div exists so that I can fade the image to black by blending it's opacity down to 0, swap it, then blend it back in.

I've looked at the computed styles in multiple browsers and can't see where the 4px delta is coming from.


Trying adding:

img { display: block; }

to your CSS. Since an <img> is an inline element by default, its height is calculated differently as related to the default line-height value.

On inline elements, the line-height CSS property specifies the height that is used in the calculation of the line box height.

On block level elements, line-height specifies the minimal height of line boxes within the element.

Source: https://developer.mozilla.org/en/CSS/line-height

Your image is using the line-height of its parent. Try this:

.imagebg { line-height: 0 }

try adding:

vertical-align: middle;

This is used in google material design lite for removing the gap between audio, canvas, iframes, images, videos and the bottom of their containers.

Material Design Lite: https://getmdl.io

Github discusion: https://github.com/h5bp/html5-boilerplate/issues/440

Basically you are getting this error on IE, though you hve not mentioned but this is the fact. IE generates some extra space below the <img> tag. Hence its a good practice to make the images img { display: block; }.

EDIT: You can say its a bug of IE

If you don't want to change display of the element, try

margin-bottom: -4px;

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