This meta issue is for any one who's interested in latest issues related to front-end development in Drupal 8 core.
There are Twig, Mobile, HTML, Design and other initiatives which have theming-related issues - her you should be able to find all this issues (meta if possible).

Novice Issues:

  1. Mobile Initiative:
  2. New Seven theme:
  3. CSS Clean up:

Intermediate Issues:

  1. Twig issues:
  2. JavaScript Cleanup:
  3. #1846970: [meta] Responsive Toolbar follow-ups
  4. Drop IE8 support

Getting started in Contributing:

  1. Drupal Core ladder (novice)
  2. Advanced patch contributor guide
  3. Drupal 8 front-end changes
  4. Coding Standards
  5. CSS Architecture
  6. IRC Channels

Best practices: