Add guideline about "Style at the lowest level element"

[hteumeuleu]

Because we don't control styles from the host of our emails, we might always end up with inheriting default styles from e‑mail clients, especially webmails. Thus a good practice is to always add relevant styles at the lowest level element.

Examples:

• Gmail and ul li
• SFR and h1, h2

[hteumeuleu]

Here’s an example from SFR webmail having the very loose CSS rule following:

div.read-content-block :not(.userSignature *) {
  font: 100% "Helvetica Neue",Helvetica,Arial,sans-serif;
  margin: auto;
  padding: 0;
}

This forces a font reset to any element that doesn't have a font declaration inline.