Build HTML first, then CSS, then JS.

Building your service using progressive enhancement will:

• ensure your service is robust and of a high quality
• make it more likely your service will work regardless of which browser or device is being used
• mean your service’s most basic functionality will work and meet the core needs of the user
• improve accessibility by encouraging best practices like writing semantic markup help users with device or connectivity limitations to use your service

and arguments against SPA.