If you use the Google Webmasters for your webpage then you most probably have seen the Mobile Usability page under the Search Traffic menu. In this subpage Google lists for you CSS, HTML or SEO errors that have to do with the mobile version of your website. One of the most common errors that is listed there is the Content wider than screen error.

After I installed the jekyll-sitemap gem on my Jekyll website and tried to register the generated sitemap in my Google webmaster account, I received from Google the following error:

Invalid tag value. This XML tag has an invalid value. Please fix it and resubmit.