[2/2] Emotion PoC – SSR Perf.

[simonsinclair]

Part 2 of bbc/psammead#3677.

Is your feature request related to a problem? Please describe.

We know from the profiling conducted in #6922 that upgrading to Styled Components v5 will grant us an additional 17% (~4 requests/sec) server throughput.

Unfortunately Styled Components v5 has a bad interaction with two companion packages we're using. These issues were raised last year and are yet to be resolved.

• Migrate to Styled Components v5 breaks snapshots styled-components/jest-styled-components#276
• sc- class appearing in snapshots when a component is created by extending another component styled-components/jest-styled-components#307

Additionally, Styled Components v4 dependencies have severe vulnerabilities.

We need to solve this as soon as possible and would like to trial a competitor CSS-in-JS library – Emotion.

Describe the solution you'd like

• Remove Styled Components and companion packages.
• Install Emotion and companion packages.
• Bump Psammead packages to RC version created in [1/2] Emotion PoC – SSR Perf. psammead#3677.
• Update SSR stylesheet rendering logic to use Emotion.

Describe alternatives you've considered

Remove jest-styled-components and babel-plugin-styled-components - meaning no styles in snapshots. This isn't really an alternative.

Testing notes

Dev insight: Will there be any potential regression? etc

• This feature is expected to need manual testing.

Additional context

n/a

[simonsinclair]

This work is complete. Please see #7772.