Why does an anchor tag's href values need http:// preprended to the URL?

Question

<a href="www.stackoverflow.com" target="_blank">click here</a>

Clicking the above link on a site's HTML page would try to take the user to

<site>/index.html/www.stackoverflow.com

Where as following works fine

<a href="http://www.stackoverflow.com" target="_blank">click here</a>

What is the rationale for this behavior?

What if you had a file named `www.stackoverflow.com` on your server? — Jeremy, Jan 25 '12 at 04:25
`http://` specifies the protocol. It's not implicit, even though web browsers have tried to make it easier for us by automatically adding it. — Cody Gray - on strike, Jan 25 '12 at 04:26
I get the point, but whats the best way to do it. If I recieve a list of urls, should I append https:// to each one of them. Or may be there is a better way to achieve it — Saurabh Tiwari, Apr 23 '19 at 10:55

score 18 · Accepted Answer · answered Jan 25 '12 at 04:27

18

There are several protocols: HTTP, HTTPS, FILE, SSH, SSL, FTP. In addition, as Jeremy mentioned, it's quite possible you may have a file on your server with the exact name of the text you're entering in the HREF.

Basically, any text in the href without a protocol is assumed to be a relative path if there is no / or protocol.

answered Jan 25 '12 at 04:27

jamesmortensen

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Or `//` to specify a path relative to the current protocol ;) – Niet the Dark Absol Jan 25 '12 at 04:30
Good point. Rarely seen, but worth mentioning for thoroughness :) – jamesmortensen Jan 25 '12 at 04:30
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Having a `/` is still a relative path, too. Just that it's relative to the domain, not the folder. If you want to be thorough, only a path that specifies the protocol too is an absolute one. ^_^ – Niet the Dark Absol Jan 25 '12 at 04:34
If you want to get technical, there are full paths, relative paths, and absolute paths. The ones beginning with `/` are full paths, yet I do agree they're relative to the domain :) I guess absolute paths are really the only ones where no matter how you look at it it's only called by one name. – jamesmortensen Jan 25 '12 at 04:37
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Something like that, anyway. Am I really this bored? I should be working! – Niet the Dark Absol Jan 25 '12 at 04:38
Lol, yeh it's easy to get addicted to Stack Overflow :) Thanks again! – jamesmortensen Jan 25 '12 at 04:43

score 9 · Answer 2 · edited Jun 20 '20 at 09:12

Update 3/2/2016:

Just use HTTPS for everything, eg: https://www.example.com

Paul Irish recommends against using protocol-relative URLs, i.e. //, and recommends writing all links using https:// - the rationale being:

Now that SSL is encouraged for everyone and doesn’t have performance concerns, this technique [protocol-relative URLs] is now an anti-pattern. If the asset you need is available on SSL, then always use the https:// asset.

Allowing the snippet to request over HTTP opens the door for attacks like the recent Github Man-on-the-side attack. It’s always safe to request HTTPS assets even if your site is on HTTP, however the reverse is not true.

Original post:

As other answers have stated, including protocols in URIs is important because otherwise you leave it up to chance how the URI is interpreted. You could have a MP4 video file literally named "www.something.com" instead of "video.mp4", or a client might try to access your website over FTP because it guessed the protocol wrong.

As Kolink pointed out in a comment, you can omit the http: entirely and just use //, eg //www.example.com. It stops mixed-content security errors ("this page has insecure elements"). It does this because browers will fetch assets such as images as if those assets were using https:// when the user is connected to the current page via HTTPS.

score 5 · Answer 3 · edited May 23 '17 at 12:17

If you leave off the protocol (http, https, ftp, mailto, etc...) the link will be relative to the current request URI. It is the same as prefixing the href with a ./ meaning this directory.

browser address bar is http://domain.com/questions/

<a href="www.stackoverflow.com" becomes domain.com/questions/www.stackoverflow.com

<a href="./www.stackoverflow.com" becomes domain.com/questions/www.stackoverflow.com

If you prefix with a / you will get from the domain root <a href="www.stackoverflow.com" becomes domain.com/www.stackoverflow.com

By adding the http:// (or any other protocol) the browsers knows this link is meant to be external to the current page. It is an absolute path because it contains the protocol and full path information.

<a href="http://www.stackoverflow.com" becomes http://www.stackoverflow.com

The link can also be protocol-relative in order to easily avoid mixed content security errors.

Furthermore, if you specify the <base href="http://domain.com/dir/"> in the (* it must be an absolute URL) Then all non-protocol prefixed URLs in href= and src= will be based on that base URL instead of the current page.

assuming the same address bar of http://domain.com/questions/ and the <base set above.

<img src="file.jpg" becomes http://domain.com/dir/file.jpg <a href="www.stackoverflow.com" becomes http://domain.com/dir/www.stacoverflow.com

score 3 · Answer 4 · answered Jan 25 '12 at 04:27

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Because the URL could have any number of protocols:

The browser doesn't know if you mean ftp://www.stackoverflow.com, http://www.stackoverflow.com or whatever, so you need to specify.

answered Jan 25 '12 at 04:27

Brill Pappin

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Is there a way to default to http like adding some attribute/flag – Rahil Ahmad Jan 25 '18 at 11:42

Why does an anchor tag's href values need http:// preprended to the URL?

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