Throw keyword density out of your mind. That is soooooo 2008.
Single page sites can be a bit of a problem. This is because if not done right, all of your content looks like a single ever changing page to the search engines which really fouls-up SEO rightly. (Sorry, I should mute SkyNews- it is bringing out the Scots in me. That's bloody well better!)
What you need to do is make sure that there are parameters to your site in such a way that allows search engines to spider your site and have each page of content look like a unique page.
Having said that, keyword density was always a bit of a myth, though not entirely. It is today and has been since the advent of Google Scholar when Scholar was written into the algorithms. Google recognized the power of citations and how recognition of citations could positively influence search in the right direction.
What still seems to hold true, though slightly modified, is the headline read order which I allude to often on this site. Here is the modern breakdown with some of my secrets included.
Create a list of your most important keywords. Create a list of your secondary important keywords. Order them from top to bottom in order of importance so that the top of each list is the most important keyword and the bottom of each list is the least important. You will be moving keywords around from one list to the other. The idea is to put them in front of you in a tangible way and not pulling them out of your mind or other area. It is a good idea to create a spreadsheet with each page represented and organize what keywords should be used and how. If you formulate a plan, you can easily modify the plan, but most importantly save the plan for future reference when tuning the site. This really helps!
Title Tag: Your top 2-3 (possibly 4) most important keywords in a conversational and compelling format. Such as Best Italian Restaurant in Cornwall Order, as much as you can, your important keywords from left to right. This is done because most people think left to right because of how we were taught to read and as a result enter search terms in that manner. Because of this phenomenon, search engines order keywords from left to right with some exception for language and keyword pairs (phrases) and historical proximity metrics. Here, you will want to match almost exactly how people will search for your site as closely as possible. In particular, now that mobile search has changed the search lexicon, search engines like close search matches with titles. Limit your
title tag to about 50 characters, though you may have to go less in order to manipulate how Google uses your
title tag. It is possible that if Google thinks your
title tag is too long (512px) it will possibly chose something else for the SERP link such as the
h1 tag for the page.
Title tag length is not the only factor however, Google may decide that it just plain does not like your title and ignore it.
Description Meta-Tag: People tend to forget about this one now that they think it carries no keyword value. But that is only half true. It actually carries a lot of keyword value in search results, but not in keyword weight for a page. Here, you want to make sure that you have a
description meta-tag that spans at least 2 lines in the SERP snippet. You can risk trying for 3 and that is fine. However, having a single line does not compel the user as much as a longer snippet but if mark-up is used, a 1 line description may be what you want. Your
description meta-tag should contain your 3-5 most important keywords plus a few from your secondary list as much as possible.
These two are your most important tags.
H1 Tag: The
h1 tag should never match your
title tag. However, it should mirror it somewhat closely. It should use 3-5 of your most important keywords and should be conversational and compelling. Use it to support your
Google uses the
title tag and
h1 tag for topic clues more than any tag excluding links. It is your 1-2 punch. These are the only two tags you can use for true weight. All others, such as image alt text, are limited in value but something to pay attention to and should not be discounted. They just are not powerful enough to really change the trajectory of a page in search performance- only complement it.
First Paragraph: Your first paragraph should reiterate essentially what the
h1 tag says but in a natural way written for users. Think of a presentation that starts off telling about the presentation, what you will learn, what will be demonstrated, and what the conclusion is. It is nearly the same thing. It is a summation of the entire page.
Link Tag: Your internal links should have your most important 2-3 keywords and should be conversational and compelling. Any inbound (back) link should also be the same but will end up being less focused. Use the URL/URI as much as possible here. There is keyword weight in the link both for the link itself, but also for the link text. Since you are using a single page site, you will have to make-up for this in your parameters. You may be losing a valuable opportunity here. Who knows until you experiment a bit.
The headline read order is this:
title tag ->
description meta-tag -> link/URL ->
header tag -> first paragraph. This is essentially a mirror of how search engines order the value of your page directly from (said to be) internal processes. I found this years ago and low-and-behold it stands true in reality even when set aside the knowledge I have of how Google operates.
Okay. Admittedly there is a bit more I can tell you on this. However this is the golden formula I use and it works aces. Drop keyword density from you lexicon and add to it headline read order and you should be fine.
But please use the single page site thing with caution. I think there will be plenty of folks with egg-foo-young on their face. I suspect this is the
thing to do de' jure and will fall out of vogue fairly quickly hitting the floor with a colossal bang!