Follow Google's progressive enhancement principles; start here:
- Content must be accessible to all browsers, as should their basic functionality
- Use semantic markup, which wraps all content
- Link your CSS & JS externally; these assets are what improves your layout & enhancing behavior
Progressive enhancement effectively deprecated Google's AJAX Crawling Specification in October 2015 (it was introduced early 2009); more about that here. What this means: Google has adopted a more progressive approach towards crawling a web page/web app. This is great news, granted you follow Google's basic SEO principles! If you don't, it won't be very hard to wrap your head around it.
Follow Google's Webmaster Principles:
- Make pages for users (not robots)!
- Don't deceive users
- Avoid tricks to improve rank (like auto generated content, hidden text, doorway pages, sneaky redirects, scraped content, etc)
- Think about what makes your website unique, valuable, or engaging. Make your website stand out from others in your field.
In short: make websites & apps for users, follow the basic principles of progressive enhancement, along with Google webmaster guidelines, and you'll do well!
Source: Core Principles of SEO for JS