A block sticks out of the column that the rest of the site lines up with.
A site that lines up on one grid looks calm and trustworthy. A block that starts a little further left or ends a little further right makes the page look messy, even when visitors cannot say why. On a phone, text that runs into the edge of the screen also reads badly.
Snaglist opens the homepage and a sample of other page types on a phone (390 pixels) and a wide screen (1440 pixels). The column of the site is the left edge of the text that comes back on almost every page, such as the header and footer. The right edge mirrors it, unless the site itself runs wider. A block outside that column is reported, unless it fills the screen, runs into the edge of the screen on purpose, or is deliberately wider in the middle. Fixed layers, sliders and hidden parts do not count.
Snaglist reports this as a notice. It costs 5 percent of the page in the health score. Good to know, and sometimes a deliberate choice.
In the report for your client this appears under the heading "Not everything lines up with your site". When everything is in order, the report says: "Everything lines up with the column of the site".
Snaglist checks every page of your site, for this issue and all the others. Start for free, no credit card needed.
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