One of the most basic things to be careful about when creating a digital book is the display settings for the user.
You have to remember that we do not know the preferences of the user who will read the book - will it be read on a low-resolution mobile phone? On a huge, high-quality tablet? In any case, it is best not to define a "margin spacing" for the text, or for any object we place on the screen. Twenty pixels can be negligible on a tablet but significant on a low-resolution phone. On top of that, if the user has set a certain margin spacing in their app - perhaps "wide margins", because that is how they like to read - the book's settings will usually be added on top of the app's settings. That harms the reading experience and can even lead to a situation where the text cannot be read at all.
Another serious problem is colors.
Sometimes, without realizing it - especially when working in automated tools like InDesign - we enter and edit text just as we are used to doing in our favorite word processor. We copy, paste, select text, mark it as bold, color it in standard black text color...
and without noticing, lines that set the text color to black are added to the CSS we attach to the book's settings. We have forgotten about our user again. What if they prefer to read at night on a black page with white text?
When we set something as fixed in our CSS, we are telling the reading app: "We always want this value. I, the book's creator, do not want this value to change."
Many books in Israel are distributed with a black text color setting, which harms the reader's ability to set colors as they wish. So at night, when switching to a black page, we get black text on a black page...
Remember - the user decides how to read their book, not the book's creator!
