Some outdated description in Introduction

[triple-underscore]

In the Introduction section, there are a few description which seems inappropriate for the current state of HTML:

In section 1.9:

HTML documents do not exist in a vacuum — this section defines many of the features that affect environments that deal with multiple pages, such as web browsers and offline caching of web applications.

The offline caching (application cache) feature has been removed from this standard.

Also, in the list of major sections in this section, the Worklets section is missing.

In section 1.7.3:

Authors can create plugins and invoke them using the embed element. This is how Flash works.

The plugin API (NavigatorPlugins etc) feature, on which Flash relies, has been removed from this standard.

[annevk]

Plugins will be dealt with as part of #6003. I can take a stab at the other issues though.