Merge branch 'master' into v4

Conflicts:
	.gitignore
	Gruntfile.js
	_config.yml
	dist/css/bootstrap-theme.css
	dist/css/bootstrap-theme.min.css
	dist/css/bootstrap.css
	dist/css/bootstrap.css.map
	dist/css/bootstrap.min.css
	dist/js/bootstrap.js
	dist/js/bootstrap.min.js
	docs/_includes/components/navbar.html
	docs/_includes/components/progress-bars.html
	docs/_includes/css/grid.html
	docs/_includes/css/overview.html
	docs/_includes/customizer-variables.html
	docs/_includes/getting-started/accessibility.html
	docs/_includes/getting-started/browser-device-support.html
	docs/_includes/getting-started/community.html
	docs/_includes/getting-started/disabling-responsiveness.html
	docs/_includes/getting-started/download.html
	docs/_includes/getting-started/examples.html
	docs/_includes/getting-started/license.html
	docs/_includes/getting-started/third-party-support.html
	docs/_includes/js/alerts.html
	docs/_includes/js/buttons.html
	docs/_includes/js/carousel.html
	docs/_includes/js/collapse.html
	docs/_includes/js/dropdowns.html
	docs/_includes/js/modal.html
	docs/_includes/js/popovers.html
	docs/_includes/js/scrollspy.html
	docs/_includes/js/tabs.html
	docs/_includes/js/tooltips.html
	docs/_includes/js/transitions.html
	docs/_includes/nav/javascript.html
	docs/_includes/nav/main.html
	docs/about.html
	docs/assets/css/docs.min.css
	docs/assets/css/src/docs.css
	docs/assets/js/customize.min.js
	docs/assets/js/raw-files.min.js
	docs/assets/js/src/customizer.js
	docs/dist/css/bootstrap-theme.css
	docs/dist/css/bootstrap-theme.min.css
	docs/dist/css/bootstrap.css
	docs/dist/css/bootstrap.css.map
	docs/dist/css/bootstrap.min.css
	docs/dist/js/bootstrap.js
	docs/dist/js/bootstrap.min.js
	docs/migration.html
	js/affix.js
	js/alert.js
	js/button.js
	js/carousel.js
	js/collapse.js
	js/dropdown.js
	js/modal.js
	js/popover.js
	js/scrollspy.js
	js/tab.js
	js/tests/unit/affix.js
	js/tests/unit/button.js
	js/tests/unit/carousel.js
	js/tests/unit/modal.js
	js/tests/unit/tooltip.js
	js/tests/visual/modal.html
	js/tooltip.js
	less/component-animations.less
	less/jumbotron.less
	less/mixins/background-variant.less
	less/mixins/buttons.less
	less/mixins/responsive-visibility.less
	less/mixins/text-emphasis.less
	less/navbar.less
	less/navs.less
	less/scaffolding.less
	less/tooltip.less
	less/utilities.less
	less/variables.less
	package.json
	scss/_buttons.scss
	scss/_forms.scss
	scss/_modal.scss

docs/getting-started/accessibility.md

@@ -5,7 +5,7 @@ title: Accessibility
 
 Bootstrap follows common web standards and—with minimal extra effort—can be used to create sites that are accessible to those using <abbr title="Assistive Technology" class="initialism">AT</abbr>.
 
-### Skip navigation
+## Skip navigation
 
 If your navigation contains many links and comes before the main content in the DOM, add a `Skip to main content` link before the navigation (for a simple explanation, see this [A11Y Project article on skip navigation links](http://a11yproject.com/posts/skip-nav-links/)). Using the `.sr-only` class will visually hide the skip link, and the <code>.sr-only-focusable</code> class will ensure that the link becomes visible once focused (for sighted keyboard users).
 
@@ -25,13 +25,13 @@ If your navigation contains many links and comes before the main content in the
 </body>
 {% endhighlight %}
 
-### Nested headings
+## Nested headings
 
 When nesting headings (`<h1>` - `<h6>`), your primary document header should be an `<h1>`. Subsequent headings should make logical use of `<h2>` - `<h6>` such that screen readers can construct a table of contents for your pages.
 
 Learn more at [HTML CodeSniffer](http://squizlabs.github.io/HTML_CodeSniffer/Standards/Section508/) and [Penn State's Accessability](http://accessibility.psu.edu/headings).
 
-### Additional resources
+## Additional resources
 
 - ["HTML Codesniffer" bookmarklet for identifying accessibility issues](https://github.com/squizlabs/HTML_CodeSniffer)
 - [The A11Y Project](http://a11yproject.com/)