Is there any browser engine or plugin which would give user information about position of given HTML element ? I want to know where is element located e.g. left corner or center of the page.
It should not be a huge problem as Firefox and Chrome marks you elements within page as you go through html code in Developer Tools > "Element tab".
Example of highlighted element : https://i.sstatic.net/Eu31L.jpg we see that selected element is currently in the centre of the screen - how to get this information programatically ?
Selenium-webdriver can give you information about any DOM element you want:
d = Selenium::WebDriver.for :chrome
d.get "http://www.google.com"
elem = d.find_element(:name, "btnI")
elem.location
=> #<struct Selenium::WebDriver::Point x=532, y=356>