Certainly, one of the most compelling reasons to consider Odoo's website builder for your CMS is that it was built, from the beginning, to support mobile devices. This feature is so central to the Website Builder application that there is a dedicated phone preview button that will let you preview what the website should look like on a mobile device:

Click the mobile preview button to see how your web page will look on a typical mobile device:

While you should still double-check all of your web pages on real mobile devices before you deploy your website, this feature is very valuable because it allows you to get an idea of what your pages will look like on mobile devices.