For a number of reasons, a long time (years) can pass from the time a patent is filed until it’s reviewed (“examined”). It can then take another few years until it’s approved. However, the patent must be examined from the perspective of it’s filing date (“priority date”). In other words, the examiner must consider the patent based on the state of technology at the time it was filed (e.g., in 2010), and not the state of technology today (2017).
So, a patent could be granted on a feature that seems very “obvious” and common today because at the time the patent was filed this feature was actually something new/innovative.
Also, there are different parts to a patent: a long “plain English” description of the technology (“specification”) and a set of “claims.” The patent is granted based on the definitions in the “claims” which is often more specific than the impression you may get reading the “specification.” So even if the “specification” sounds like it’s describing a well-known feature (Shadow DOM, email, the internet, etc.), the actual “claims” may cover something much more specific (e.g., a minor improvement to the feature).