Use css contain to help browser perfs

[dascritch]

References :

• https://developers.google.com/web/fundamentals/web-components/shadowdom
• https://developer.mozilla.org/en-US/docs/Web/CSS/contain
• https://developers.google.com/web/updates/2016/06/css-containment
• https://termvader.github.io/css-contain/
• https://blogs.igalia.com/mrego/2019/01/11/an-introduction-to-css-containment/

This should be suggestion for users for integrating web component.

Suggested in first link :

:host {
  display: block;
  contain: content; /* Boom. CSS containment FTW. */
}

also

:host {
   all: initial; /* 1st rule so subsequent properties are reset. */
}

so we can remove reset styles (see https://developer.mozilla.org/fr/docs/Web/CSS/all )

[dascritch]

:host properties added (due to another bug https://github.com/dascritch/cpu-15/issues/20 ). We just need some cleanse in css.