Is it valid to have a html form inside another html form?

Is it valid html to have the following:

<form action="a">
    <input.../>
    <form action="b">
        <input.../>
        <input.../>
        <input.../>
    </form>
    <input.../>
</form>

So when you submit “b” you only get the fields within the inner form. When you submit “a” you get all fields minus those within “b”.

If it isn’t possible, what workarounds for this situation are available?