```python
"""Report the stretches of a clinic's day that no appointment covers."""

import re


def open_windows(span: str, booked: list[str]) -> str:
    def minutes(text: str) -> list[int]:
        parts = text.split("-")
        if len(parts) != 2 or any(re.fullmatch(r"\d{2}:\d{2}", p) is None for p in parts):
            raise ValueError("a range must be written HH:MM-HH:MM: " + text)
        return [int(p[:2]) * 60 + int(p[3:]) for p in parts]

    def stamp(at: int) -> str:
        return f"{at // 60:02d}:{at % 60:02d}"

    span_start, span_end = minutes(span)
    
    # Parse and validate all booked appointments
    intervals = []
    for b in booked:
        start, end = minutes(b)
        # Appointments must be inside the span
        if start < span_start or end > span_end:
            raise ValueError("appointment outside span")
        intervals.append((start, end))
    
    # Sort by start time
    intervals.sort(key=lambda x: x[0])
    
    free: list[str] = []
    current = span_start
    
    for start, end in intervals:
        if start > current:
            free.append(f"{stamp(current)}-{stamp(start)}")
        # Update current to the end of the current appointment
        if end > current:
            current = end
            
    # Add the remaining free time after the last appointment
    if current < span_end:
        free.append(f"{stamp(current)}-{stamp(span_end)}")
        
    return ", ".join(free) if free else "none"
```