Some warning in Visual Studio 2015 working whit HTML5, CSS3 and razor

Question

Why gives Visual studio Professional 2015 this warnings on code below? I don't see any problem.

• Element 'button' requires end tag. (on first line)
• End tag is missing matching start tag. (on last line)
• Validation (CSS 3.0): "flex" is not a valid value for the "position" property. (on second line by flex)

<button type="submit" class="btn btn-default flex-button external-authentication" 
        id="@p.AuthenticationType" style="position: flex;" name="provider" 
        value="@p.AuthenticationType" title="Meld u aan met uw @p.Caption account">
    <p>
        <span class="fa-stack fa-lg fa-2x" style="color: @kleur;">
            <span class="fa fa-square fa-stack-2x">
            <span class="@icon fa-stack-1x fa-inverse">
        </span>
    </p>
</button>

^{The code isn't changed between the code in my solution and what you see here.}

Answer 1

I've found it by @Alohci comment. He or she sayd:

• button elements cannot contain p elements.
• flex is a value of the display property, not the position property.