Where to specify image dimensions for fastest rendering: in HTML or in CSS?

I've learned that it is a best practice to explicitly specify image dimensions. The browser can then already layout the page while still downloading the images themselves, thereby improving (percieved) page rendering time.

Is this true? And if so, is there a difference in specifying the开发者_开发百科 dimensions in either HTML or CSS?

• HTML: <img src="" width="200" height="100">
• Inline CSS: <img src="" style="width: 200px; height: 100px">
• External CSS: #myImage { width: 200px; height: 200px; }

According to Google Page Speed, it does not really matter if you specify the dimensions via CSS or HTML, as long as your CSS targets the IMG tag itself and not a parent element :

When the browser lays out the page, it needs to be able to flow around replaceable elements such as images. It can begin to render a page even before images are downloaded, provided that it knows the dimensions to wrap non-replaceable elements around. If no dimensions are specified in the containing document, or if the dimensions specified don't match those of the actual images, the browser will require a reflow and repaint once the images are downloaded. To prevent reflows, specify the width and height of all images, either in the HTML tag, or in CSS.

However, note that they advise not to resize the image using these dimensions, ie to always use the real dimensions :

Don't use width and height specifications to scale images on the fly. If an image file is actually 60 x 60 pixels, don't set the dimensions to 30 x 30 in the HTML or CSS. If the image needs to be smaller, scale it in an image editor and set its dimensions to match (see Optimize images for details.)

I tend to do it in the CSS. This is certainly a win when there are multiple images with the same dimensions (so you can do stuff like .comment img.usergroup { width: 16px; height: 16px; }), and have the same images subject to scaling in different stylesheets like user-selectable themes.

When you have completely independent images that are used on the site only once, it doesn't really make sense to abstract their size out to CSS, so the HTML version would probably be more appropriate there.

I think CSS gives you more flexibility: you can specifically set the width or height while setting the other dimension to auto. But when setting both dimensions, I don't thing there's a difference.

This does not answer your question directly, but I would not rely on the dimensions of your image for page layout. Instead include the image in a block level element. This relieves both the HTML and CSS from having to hold information that it really shouldn't as the image may change from time to time.

If you put a large image in an HTML page without dimensions, you should definitely notice the page layout shifting as the image is downloaded (over an internet connection, if not locally).

As per other answers, it doesn’t make much difference whether you do this in the HTML or the CSS.