Javascript:

No react lifecycle hooks (only in 5% cases we will allow and that too with very little scripting).
Components should have 90% html.
Refer examples of coupon pages for mobile and desktop when in doubt. We have to get same smooth speed across complete site.
Scripting business logic allowed only in Actions.
use React.PureComponent with no extraneous props, always (Refer my latest changes in performance optimization).


CSS guidlines:

Image: should have exact size in html with placeholder during load.
No browser specific prefixes.
keep 0 specificity and style with classes only. Never style elements directly.