Self-referential hreflang attributes for single language website?

There is no benefit in using the hreflang on a single language website, it is meant exclusively for multilingual and international websites. I am not suggesting that Google (or other search engines) would penalize you for it, but they would definitely not reward you.

You can see a proper implementation of the hreflang tag over at trip advisor - a well respected multilingual and global brand (which includes both country and language targeting) below and you should read the following article and associated official Google Video publised a few days ago:

http://www.seroundtable.com/googles-international-targeting-webmaster-tools-18782.html https://www.youtube.com/watch?v=z51qbs7wLx8&feature=youtu.be&t=22m40s

Bottom line: Do not add the tag to single language website

The sampled Trip Advisor code (where en-GB is targeting English in the UK and en-CA targeting English speakers in Canada) :

<link rel="alternate" hreflang="en" href="http://www.tripadvisor.com/"/>
<link rel="alternate" hreflang="en-GB" href="http://www.tripadvisor.co.uk/"/>
<link rel="alternate" hreflang="en-CA" href="http://www.tripadvisor.ca/"/>
<link rel="alternate" hreflang="it" href="http://www.tripadvisor.it/"/>
<link rel="alternate" hreflang="es" href="http://www.tripadvisor.es/"/>
<link rel="alternate" hreflang="de" href="http://www.tripadvisor.de/"/>
<link rel="alternate" hreflang="fr" href="http://www.tripadvisor.fr/"/>

BTW - Google does not need the lang element as well, it reads a site's language just fine. If you want to target a specific English speaking country (for example), set your preferred country in Google webmaster tools. The lang element is a W3C recommendation and not Google's - they can associate a site with a proper language for quite sometime now:

http://googlewebmastercentral.blogspot.co.il/2010/03/working-with-multilingual-websites.html

There are no tangible benefits from using the hreflang attribute, beyond special usage like the one described by Google. It is declarative markup and does not cause any action or affect rendering, unless you make it to. The HTML5 LC explicitly warns: “It is purely advisory. [...] User agents must not consider this attribute authoritative — upon fetching the resource, user agents must use only language information associated with the resource to determine its language, not metadata included in the link to the resource.”

It is thus a bit of a mystery what the attribute is meant to be used for, except in link elements with rel="alternate". Browsers might communicate the information to the user, at least when the user asks for information about a link. But such things, though potentially useful, have been very limited in browsers.

The specific element presented in the question says that there is an alternate version of the current document in English language at http://www.example.com. The href attribute is rather pointless if the current document is in English. And if it is an another language, you actually have a page in two languages-