<input type=checkbox>: single and grouped

Two checkbox patterns

Single — boolean opt-in:

<input type="checkbox" id="tos" name="accepted_tos" value="yes" required />
<label for="tos">I agree to the terms</label>

If checked: submits accepted_tos=yes. If unchecked: nothing is submitted at all (the key just doesn't appear in the body). required forces the user to check.

Group — multi-select:

<input type="checkbox" id="py" name="languages" value="python" />
<label for="py">Python</label>

<input type="checkbox" id="js" name="languages" value="javascript" />
<label for="js">JavaScript</label>

<input type="checkbox" id="go" name="languages" value="go" />
<label for="go">Go</label>

Same name, different value. Submitted as: languages=python&languages=javascript (if first two checked).

FastAPI receives a list

async def submit(languages: list[str] = Form([])):
    return {"langs": languages}     # ["python", "javascript"]

The empty list [] default is critical — if zero boxes checked, the key isn't in the body at all and FastAPI errors out without a default.

checked attribute

Pre-check on first render:

<input type="checkbox" name="newsletter" value="yes" checked />