This is something you need to pay particular attention with when switching themes. There are certain themes that have features where you can add the meta description, title tags, etc from the theme backend.
This is all fine and dandy, but if you throw an SEO plugin such as Yoast or SEO ultimate into the mix and you’ve got issues. Always verify that you don’t have duplicate tags by checking the source code of multiple pages.The source code doesn’t lie.
Sure Google Webmaster Tools or your favorite neighborhood SEO plugin might give you a warning, but by then it is too late and Google has crawled your site and has taken notes. I’ve seen lots of sites fall into this trap in the past. I’ve even seen sites so mis-managed that they had duplicates x 3 within the source code.
Scenario: your manager just approved your new design, HR loves it, and so does your Mom. Its flashy, has that cool scrolling effect thingy, has multiple forms on the homepage and a slider everyone can agree on. Problem is, it takes 12 second to load and is 2000 lines of code.
Be very weary of themes that will bloat your site. Not only is it bad for SEO, but it is horrible for user engagement as well. In general, people hate waiting.
We’ve written extensively on the correlations between website speed and SEO and even wrote a guide to WordPress site speed guide. Watch out for themes from places like Themeforest. This is not to say they are all bad, but they can get bloat-ey at times.
Stick with known frameworks such as the Genesis framework. If you aren’t sure, go to the demo site and play with it before installing. Run it through tools.pingdom.com and see how many requests it has, how long it takes to load, and how many lines of code it takes to get the job done.
Also watch out for things like shortcodes. While this might make creating CSS much easier, it adds a lot of bloat to your CSS file.
It is no secret structured data and SEO are growing closer and closer, but we all still have a lot to learn. There are many different themes that attempt to implement Schema within the core theme files. While this is great, it isn’t always implemented properly. Just the other day I noticed an issue where the theme on my personal website rendered 2 “WebPage” Schema types which caused a number or validation issues.
This is also true for open graph tags, hreview tags and lots of other semantic data. Google is depending on this data to build its Knowledge Graph, so don’t screw it up!
Nulled themes aka ‘hacked themes’ are just plain stupid. Story time: One time I paid a developer to customer a theme, who took a shortcut and used a nulled theme to try and save a few bucks. Begin catastrophe.
Turns out that theme had a php backdoor, which compromised our FTP dev server, which compromised our server and caused 2 days worth of downtime for 50 domains and dozens of hours of development. In short, don’t be an idiot, but your themes. Not only is this illegal, but it is unethical and really not nice. Most of these developers rely on these small fees to support themselves. Not cool.
Beyond being generally uncool, hacked themes are notoriously bad for SEO. Hackers embed all kinds of nonsense into these themes including affiliate id’s, backlinks, redirect your entire blog and all sorts of other stuff. This goes for nulled plugins as well, which are just as bad. Stay away from them, it is just not worth it.
Many themes come responsive ready out of the box. Other themes claim to be, but are actually not. For me, the only way to convince me that a WordPress theme is good to go is to see that it passes Google’s mobile friendly tool. This is a somewhat stringent test, and might not be accurate 100% of the time but the only opinion I really care about is Google’s.
What I like to do whenever launching a new WordPress site / theme is to remove myself from the equation. Have 2 of your closest SEO confidants look at the theme and tear it apart. Add lots of sample data to see what it will look like when the site is full. Add some users, posts, pages, images, and custom post types. Always check reviews of the themes you are buying as well. Not just the ones that say the theme is “nice,” but the ones from skilled webmasters with some code under their belts. Good luck!
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Part 1 - An SEO's Guide to Tumblr
Part 3 - The value of Tumblr links revisited