#1 is about planning for extensibility. Just look at the hackery with JS where lonely, otherwise ignored, strings are used for things like "use strict" and "use asm". Or where Microsoft added "conditional comments", which quite frankly, was essential to the development of Outlook Web Access, which basically gave us Ajax. Or all the absurd vendor prefixes on CSS tag names. Or one of 100 other little hacks that browser vendors have invented to try to innovate past the standard. Pushing pass the standard, by the way, is the only way forward. We've learned that lesson by now, so we should plan for extensibility.