Describe the bug
When running Deque's Axe plugin on a webpage, I found multiple accessibility errors with the Storybook boilerplate.

To Reproduce
Steps to reproduce the behavior:

1. Load and run the Storybook boilerplate with generic components - Button with Text and Button with Emoji.
2. Download Deque's Axe Google Chrome plugin
3. Run the plugin on the running boilerplate.
4. See accessibility errors with exact selectors where issues were found.

Expected behavior
Axe should return no errors.

System:
Binaries:
Node: 10.15.3
npm: 6.4.1
npmPackages:
@storybook/cli: 5.3.17
@storybook/html: 5.3.17

Additional context
Some issues have already been opened with solutions to the problems within Storybook's boilerplate. I've linked to those, where applicable. I have also linked directly to Deque's documentation on the required fixes for each accessibility defect.

1. Page must have means to bypass repeated blocks.
   

   • Previously-opened issue with this same problem: #4667

2. Page must have one main landmark.
   

3. Page must contain a level-one heading.
   

4. All page content must be contained by landmarks.
   

5. Text Elements must have sufficient color contrast against the background

   • Previously-opened issue with this same problem: #8935

Running the plugin before every release will help validate that no accessibility bugs exist and that consumers are compliant when building out storybook applications. Thank you for your time!