How to get high resolution website logo (favicon) for a given URL

The Go code at https://github.com/mat/besticon tries to solve this problem.

For example

$ besticon http://github.com 
http://github.com:  https://github.com/apple-touch-icon-144.png

There is also an accompanying hosted version of the code, see for example http://icons.better-idea.org/icons?url=github.com.

(Disclaimer: I wrote it because I needed to solve the same problem a while ago.)

You can code it yourself or use an existing solution.

Do-it-yourself algorithm

1. Look for Apple touch icon declarations in the code, such as <link rel="apple-touch-icon" href="/apple-touch-icon.png">. Theses pictures range from 57x57 to 152x152. See Apple specs for full reference.
2. Even if you find no Apple touch icon declaration, try to load them anyway, based on Apple naming convention. For example, you might find something at /apple-touch-icon.png. Again, see Apple specs for reference.
3. Look for high definition PNG favicon in the code, such as <link rel="icon" type="image/png" href="/favicon-196x196.png" sizes="196x196">. In this example, you have a 196x196 picture.
4. Look for Windows 8 / IE10 and Windows 8.1 / IE11 tile pictures, such as <meta name="msapplication-TileImage" content="/mstile-144x144.png">. These pictures range from 70x70 to 310x310, or even more. See these Windows 8 and Windows 8.1 references.
5. Look for /browserconfig.xml, dedicated to Windows 8.1 / IE11. This is the other place where you can find tile pictures. See Microsoft specs.
6. Look for the og:image declaration such as <meta property="og:image" content="http://somesite.com/somepic.png"/>. This is how a web site indicates to FB/Pinterest/whatever the preferred picture to represent it. See Open Graph Protocol for reference.
7. At this point, you found no suitable logo... damned! You can still load all pictures in the page and make a guess to pick the best one.

Note: Steps 1, 2 and 3 are basically what Chrome does to get suitable icons for bookmark and home screen links. Coast by Opera even use the MS tile pictures to get the job done. Read this list to figure out which browser uses which picture (full disclosure: I am the author of this page).

APIs and open source projects

RealFaviconGenerator: You can get any web site favicon or related icon (such as the Touch Icon) with this favicon retrieval API. Full disclosure: I'm the author of this service.

BestIcon: Although less comprehensive, Besticon offers a good alternative, especially if you want to host the code yourself. There is also a hosted version you can use right away.