But hardly any book or text today is written in HTML, not to speak of handcoded PDF. Rather, text-processors ranging from rather archaic to high-tech are used. Some of these text-processors do offer converting into HTML and/or PDF, and some don't, so one probably has to go a more complicated way in reaching the wanted target format. And even if there's a straight way for conversion, today's tools aren't always 100% perfect. Bugs and immaturity often prevent correct conversion into the target format in a single step, so it's often necessary to help these tools, sometimes even fixing what they broke.