The essence is, there is a samopisny widget, embedded by js in the <head> . If the site has a standard container class, then everything is fine, but if not, then the widget elements can be shifted horizontally. For a little clarity (very roughly): 
It is required that the border always coincides with the boundary of the content, how can this case be calculated?
Tried to calculate the width first div in body $('body > div').css("width"); but there may be anything, I would like a universal and reliable solution. Gave a link to the curve of the site, it is necessary that even on it normally displayed ( curve site ).