chore: install w3c html validator
feat: use xhtml strict doctype
fix: remove bugged gulp-cache plugin
feat: enhance palette ratios
refactor: use balise ends trick instead of comments trick

- it's less verbose
- also remove "S'inscrire" link
refactor: use generic baseline for less
fix: stick menu items to the navbar
fix: use 100% width a link for the menu

- also reduce less selector nesting
feat: create event content
feat: play with menu opacity