Why doesn't look my slider exactly the same in every browser/device?

The position of the layers, font sizes are calculated by javascripts. That javascript is calculating as exactly, as it can, to make it look smooth, when you make your slider smaller, not just to jump between sizes. For example your text’s size could be 4.58036px, but it’s up to the browser, if it will show the text with this exact number, or round it up or down, and with the same value, it still shows it a little differently. This is only one text item, but there are the layers too, which are calculated from the size of the slider. The point is, that there could be some pixel differences, and you shouldn’t count on not having them. Here is a simple example, how much difference there are between browsers:
fonts