Although it is possible to precisely control the position of elements with any of the absolute units, pixels are the most natural way to define measurements for screen based media. Despite being a “relative” size, pixels behave absolutely in context to the screen resolution, and many modern Web designs will be specified in pixels because it is the most universal measurement regardless of screen size, OS, or browser. Like atoms in molecules, pixels are irreducible as the smallest unit on the screen—you can’t move something half a pixel.
While pixels give the designer precise control over where elements appear, they are not without their issues and inconsistencies. Most Web browsers allow users to enlarge text and zoom the page size, which is imperative for anyone with poor vision. Locking the font size with pixels or absolute values prevents Internet Explorer from changing their size. Internet Explorer 7 goes some way in rectifying this limitation by allowing for the entire page to be zoomed, but there is still some debate over their use when accessibility is an issue. So, by using pixels you get precision in your design at the sacrifice of versatility.
In web design, fixed layouts are those that use a specific unit of measurement to define web page widths – independent of all other factors, including browser window dimensions and font sizes. In other words, the webpage/site will be the same fixed sized regardless of the size of the browser window or the size of font in which the page is written.With relative layouts, column widths are defined to fill up a certain fraction of the browser window, regardless of the actual size of the window on which the page is viewed. If a viewer expands or contracts the website to be bigger or smaller, the columns will automatically resized so that their size, relative to the size of the browser window, remain the same.(http://www.onextrapixel.com/2015/07/13/the-definition-of-layouts-in-web-design-and-when-to-use-them/)
For instance, the main column of a webpage/site can be fixed at 740 pixels. If a viewer accesses that webpage on a minimized browser window, the main column would still be 740 pixels wide, and this would not change regardless of how big or small the browser window becomes. This implies that if the browser window is smaller than the fixed width of the page, a viewer would be unable to see everything on the page all at once.(http://www.onextrapixel.com/2015/07/13/the-definition-of-layouts-in-web-design-and-when-to-use-them/)