Having 15 years of experience in web designing is a significant accomplishment and indicates a wealth of knowledge and skills in the field. With such extensive experience, you are likely to have a deep understanding of various aspects of web design, including:

1. HTML, CSS, and JavaScript: Proficiency in these core technologies for building and styling web pages.

2. Responsive Design: Knowledge of creating websites that adapt and display well on various devices, such as desktops, tablets, and smartphones.

3. User Experience (UX) Design: Understanding how to create intuitive and user-friendly interfaces to enhance the overall user experience.

4. Graphic Design: Ability to create visually appealing layouts and graphics for websites.

5. Web Design Tools: Familiarity with popular web design tools like Adobe Creative Suite (Photoshop, Illustrator, etc.) or other design software.

6. Cross-Browser Compatibility: Knowledge of making websites work consistently across different web browsers.

7. Search Engine Optimization (SEO): Understanding how to optimize websites to improve their search engine rankings.

8. Web Performance Optimization: Techniques to enhance website loading speed and overall performance.

9. Web Accessibility: Knowledge of designing websites to be accessible to people with disabilities.

10. Version Control Systems: Familiarity with version control tools like Git for collaborative web development.

11. Web Design Trends: Staying up-to-date with current design trends and best practices.

Your experience could also involve working on various projects, such as e-commerce websites, corporate sites, blogs, portfolios, and more. With such a diverse background, you may have encountered and resolved numerous challenges and gained insights into how to create effective and impactful web designs.