updating CSS docs to reference main.css (#2079)

Also, o need to dupe this documentation over two repos.

src/doc/css.md

@@ -39,7 +39,7 @@ page](https://necolas.github.com/normalize.css/), as well as this
 [blog post](http://nicolasgallagher.com/about-normalize-css/).
 
 
-## Useful defaults
+## main.css
 
 Several base styles are included that build upon `Normalize.css`. These
 styles:
@@ -49,116 +49,6 @@ styles:
 * tweak the default alignment of some elements (e.g.: `img`, `video`,
   `fieldset`, `textarea`)
 * style the prompt that is displayed to users using an outdated browser
+* and more...
 
-You are free and even encouraged to modify or add to these base styles as your
-project requires.
-
-
-## Common helpers
-
-Along with the base styles, we also provide some commonly used helper classes.
-
-#### `.hidden`
-
-The `hidden` class can be added to any element that you want to hide visually
-and from screen readers. It could be an element that will be populated and
-displayed later, or an element you will hide with JavaScript.
-
-#### `.visuallyhidden`
-
-The `visuallyhidden` class can be added to any element that you want to hide
-visually, while still have its content accessible to screen readers.
-
-See also:
-
-* [CSS in Action: Invisible Content Just for Screen Reader
-  Users](https://webaim.org/techniques/css/invisiblecontent/)
-* [Hiding content for
-  accessibility](https://snook.ca/archives/html_and_css/hiding-content-for-accessibility)
-* [HTML5 Boilerplate - Issue #194](https://github.com/h5bp/html5-boilerplate/issues/194).
-
-__N.B.__ [The visuallyhidden class can be an accessibility issue for users using high contrast modes.](https://www.paciellogroup.com/blog/2012/08/notes-on-accessible-css-image-sprites/)
-
->Use JavaScript to detect when images are disabled and remove the CSS visually hidden display state of the text alternative.
-Use JavaScript to detect when Windows high contrast mode is enabled and remove the CSS visually hidden display state of the text alternative.
-
-#### `.invisible`
-
-The `invisible` class can be added to any element that you want to hide
-visually and from screen readers, but without affecting the layout.
-
-As opposed to the `hidden` class that effectively removes the element from the
-layout, the `invisible` class will simply make the element invisible while
-keeping it in the flow and not affecting the positioning of the surrounding
-content.
-
-__N.B.__ Try to stay away from, and don't use the classes specified above for
-[keyword stuffing](https://en.wikipedia.org/wiki/Keyword_stuffing) as you will
-harm your site's ranking!
-
-#### `.clearfix`
-
-The `clearfix` class can be added to any element to ensure that it always fully
-contains its floated children.
-
-Over the years there have been many variants of the clearfix hack, but currently,
-we use the [micro clearfix](http://nicolasgallagher.com/micro-clearfix-hack/).
-
-
-## Media Queries
-
-HTML5 Boilerplate makes it easy for you to get started with a
-[_mobile first_](https://www.lukew.com/presos/preso.asp?26) and [_responsive web
-design_](https://alistapart.com/article/responsive-web-design) approach to
-development. But it's worth remembering that there are [no silver
-bullets](https://cloudfour.com/thinks/css-media-query-for-mobile-is-fools-gold/).
-
-We include placeholder media queries to help you build up your mobile styles for
-wider viewports and high-resolution displays. It's recommended that you adapt
-these media queries based on the content of your site rather than mirroring the
-fixed dimensions of specific devices.
-
-If you do not want to take the _mobile first_ approach, you can simply edit or
-remove these placeholder media queries. One possibility would be to work from
-wide viewports down, and use `max-width` media queries instead (e.g.:
-`@media only screen and (max-width: 480px)`).
-
-
-## Print styles
-
-Lastly, we provide some useful print styles that will optimize the printing
-process, as well as make the printed pages easier to read.
-
-At printing time, these styles will:
-
-* strip all background colors, change the font color to black, and remove the
-  `text-shadow` — done in order to [help save printer ink and speed up the
-  printing process](http://www.sanbeiji.com/archives/953)
-* underline and expand links to include the URL — done in order to allow users
-  to know where to refer to<br>
-  (exceptions to this are: the links that are
-  [fragment identifiers](https://developer.mozilla.org/en-US/docs/Web/HTML/Element/a#attr-href),
-  or use the
-  [`javascript:` pseudo protocol](https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Operators/void#JavaScript_URIs))
-* expand abbreviations to include the full description — done in order to allow
-  users to know what the abbreviations stands for
-* provide instructions on how browsers should break the content into pages and
-  on [orphans/widows](https://en.wikipedia.org/wiki/Widows_and_orphans), namely,
-  we instruct
-  [supporting browsers](https://en.wikipedia.org/wiki/Comparison_of_layout_engines_%28Cascading_Style_Sheets%29#Grammar_and_rules)
-  that they should:
-
-  * ensure the table header (`<thead>`) is [printed on each page spanned by the
-    table](http://css-discuss.incutio.com/wiki/Printing_Tables)
-  * prevent block quotations, preformatted text, images and table rows from
-    being split onto two different pages
-  * ensure that headings never appear on a different page than the text they
-    are associated with
-  * ensure that
-    [orphans and widows](https://en.wikipedia.org/wiki/Widows_and_orphans) do
-    [not appear on printed pages](https://css-tricks.com/almanac/properties/o/orphans/)
-
-The print styles are included along with the other `css` to [avoid the
-additional HTTP request](http://www.phpied.com/delay-loading-your-print-css/).
-Also, they should always be included last, so that the other styles can be
-overwritten.
+These styles are included in [main.css](https://github.com/h5bp/html5-boilerplate/blob/master/dist/css/main.css). See the [main.css](https://github.com/h5bp/main.css) project [documentation](https://github.com/h5bp/main.css/blob/master/README.md#features) for a full discussion of these styles.