Responsive HTML Modification

I need an experienced web developer to modify existing HTML code for a private website. The modifications should ensure responsive web design for both smartphones and tablets, and add placeholder mouseover text (e.g., "hotspot1", "hotspot2") on an included image map. Cannot use Google APIs.

Requirements:
- Modify HTML for responsive design
- Add placeholder mouseover text on image map
- Ensure compatibility with smartphones and tablets

Ideal Skills:
- Proficient in HTML
- Experience with responsive web design
- Attention to detail for accurate mouseover text placement

No CSS framework is currently in use. Looking for clean and efficient code. Please provide examples of similar work and estimated completion time.