Mobile SEO factors that matters

Mobile SEO is largely technical optimization of a website for mobile use so that its usability is recognized by search engines.

Mobile usability is determined by the accessibility and the website performance on a mobile device.

Main features are:
(1) no horizontal scroll bar
(2) no pop-ups
(3) clickable elements are not too close to each other
(4) the website can be used with a slower internet connection
(5) the readability of text has been taken into account

Make sure that the website requires as little data as possible and that the data is instant available.

The greatest gains can be achieved by:
(1) Minimize the number of external elements on your website (images, style sheets and scripts)
(2) Optimize images and video so that they have a minimum size
(3) Using a static website or a website environment which supports caching

Your website will work on any device when you have a fast website with a single column layout and a responsive design.

Use one web page for one topic. Use headlines, write short sentences and use white space.

For the headlines it is adviced to follow these usage notes.

Google and Bing have established guidelines for mobile websites: (Mobile-friendliness (Google), Basics of UX (Web.dev), The Bing Mobile Friendliness Test Tool (Bing))

Perhaps most importantly, the website should display the same content on the mobile device as it does on a computer. This is because only a mobile web crawler is used.

The advice for website designers is therefore: Design for mobile!

Structured data adds value to mobile SEO because a mobile device has limited screen space.

Hence, a search result with rich snippets stands out more on a mobile device than on a desktop.

You can view the structured data at your website with ZOMDir's Page Inspector and test this data with Google's Rich Results Test.

Search engines such as Google and Bing offer a tool to test whether your website is considered mobile-friendly.

These tools are:
Google's PageSpeed Insights (Note that the original Mobile-Friendly Test is gone)
Bing's Mobile Friendliness Test Tool

Automatic accessibility tests test some accessibility requirements.

However, these tools do give a good first impression of the accessibility of your website.

You can use the following tools:
Web Accessibility Checker (Gone)
European Internet Inclusion Initative
WAVE

There are a lot of advices and tools regarding your website speed. The most important tools to test your website speed are:
Google's PageSpeed Insights
GTmetrix
Pingdom Website Speed Test