A number of programming languages. Source control. Integrated Development Environments (IDEs). Object-Oriented Principles (OOP) and best practices.
Beautiful web sites. Mobile-first design. Single Page Applications (SPAs). Responsive design. Great performance. Search Engine Optimization (SEO). A variety of hosting services.
REpresentational State Transfer (REST). API creation and consumption. Microservices architecture. JSON. SOAP. XML. You name it.
Create, read, update, and delete (CRUD) operations. Performance optimization. Database normalization. Other good things.
Identity and Access Management (IAM). Encryption. Cryptography. OWASP Top 10 threat mitigation. Sleep better at night.