Fix HTML for better readability

 * Use #wrapper from mozfr.css to avoid a full-width body
 * Remove all spurious spaces
 * Use <p> where appropriate to have a similar text in HTML and browser
 * Display by default all specific instructions to avoid confusing JS
 * Minimal changes to copy text
Fix typos
Perform all-caps in CSS rather than in HTML
Improve copy text
Use the same tense everywhere
Clarify after a full read