Mobile is becoming more complicated with the increasing popularity of tablets and mobile phones. As such, SEO is getting more complicated (primarily in regards to URLs). Google has tried to simplify this by getting behind responsive design and having one URL and one version of your content, but I have a hard time getting behind this as a widespread solution (more on this below). Though this post contains guidelines for mobile SEO, keep in mind that there are always exceptions to the rule and sites should deviate from best practices.
While Google recommends responsive design for mobile, having the same content and same URL for all users; I recommend having one URL that dynamically serves different content based on whether the user agent is a desktop or mobile device. It may be worth even serving different versions of domain.com to mobile users based on whether they are on a tablet or smart phone.
This is a better solution than responsive design (as a whole solution) as desktop, tablet, and smart phone users can all have very different needs – I think that very few users will make significant and high value purchases on their phone (though they may well do research), but users are very likely to on a desktop and it’s definitely possible on a tablet. I think that this warrants different content and experiences for different types of users.
I prefer a consistent URL over m.domain.com as having a consistent URL will consolidate all link equity on one URL and maximize your link equity. People are less likely to link to mobile content, so link equity flow from mobile pages to desktop pages is not a concern, but having one URL allows your desktop link equity to flow to your mobile site.
When you use a single URL and dynamically change the content, it is important to use the “Vary HTTP Header”. This signals to Google that the content here changes based on the user agent; as such they will know that the content may be different for mobile and can crawl with their mobile bot as well. Google has provided some documentation on this here (about halfway down the page)
Google has a good overview on implementing user agent detection and redirecting users here.
Advice for Using a Mobile Specific URL
Redirecting Mobile Users
If you end up use a mobile specific URL (m.domain.com), you will need to redirect users based on their user agent. You should do this via a HTTP header status code. Google has said that it does not matter if you use a 301 or 302 redirect for this.
You need to make use of the rel=”alternate” tag. On the desktop page, you need to add the rel=”alternate” tag and point to the mobile version of the page. This will help Googlebot to discover the mobile content. This can be done either in the <head> section of the web page or the xml sitemap. You can find more on rel alternate here and here.
Again, if you use the mobile specific URL (m.domain.com), you will need to set the canonical tag of the mobile page to the desktop page URL. For example, the canonical tag for m.domain.com/slug would be set to “www.domain.com/slug”.
Overall, you want to be carrying over the same principles of onsite SEO from your desktop pages to your mobile pages.
Page Titles for Mobile
In most cases, you should be using the same title for mobile and desktop content. The one thing to keep in mind here is that mobile is inherently a smaller screen. Titles will probably show up properly for tablets but most likely not on phones.
For this reason, for smart phones (or mobile if you don’t separate tablets and phones), you should format your titles as follows since you have a recognizable brand:
Brand Name | Page Title/Keyword Phrase
As with the desktop versions of pages, you should have a large headline on every page that has your primary keyword phrase on it. This should be an H1 tag (and the only one on the page). I know a lot of people argue that the H1 is no longer a factor, but these arguments are based on correlation studies which say that the H1 is not very strongly correlated with higher rankings, not that it isn’t a factor. I still believe the H1 tag is a ranking factor.
Onpage SEO Copy Guidelines for Mobile
For desktop pages, I recommend about 150 words minimum of onpage copy that is targeted and focused on your primary keyword/category for the page(not all text on the page: does not include reviews, product descriptions, etc). For mobile, we still want to have a good block of text, but we should take into consideration the user’s smaller screen. For this reason, I think 50-75 words is a good starting point for mobile users. If you are going to break out phones and tablets, 50-75 words is good for phones; with tablets, this number can be higher, it would probably be fine to give tablet and desktop users the same content here.