How to create a div with a diagonal (or angled) top line

Question 1

First I thought this could be done with CSS triangles, but I can't quite work out how to make a downward right pointing triangle be 100% width, so I moved on to another option...

You could keep using the rotation technique as you are doing, but then add some padding to the bottom of the green part, and then use a negative margin-bottom to bring the footer back up and over the diagonal. If you then set the footer to relative, it should sit over the top of the green so you won't see the diagonal line.

I tested this on the actual site and found that I need to add the padding to the container inside footer-top and put the negative margin on footer-top. Otherwise, the padding would cause a gap at the bottom of the body.

Hard to describe in words, so here's the snippet.

#footer-top {
    color: #fff;
    padding: 35px 0 15px;
    transform:  rotate(2deg) scale(1) skew(3deg) translate(0px);-webkit-transform:  rotate(2deg) scale(1) skew(3deg) translate(0px);-moz-transform: rotate(2deg) scale(1) skew(3deg) translate(0px);-o-transform:  rotate(2deg) scale(1) skew(3deg) translate(0px);-ms-transform:  rotate(2deg) scale(1) skew(3deg) translate(0px);
    
    margin-bottom: -20px; /* Added */
}

/* Added */
#footer-top .container {
    padding-bottom: 20px;
}

#footer-space {
    padding: 0 0 6px;
}

#footer-top2 {
    color: #fff;
    background-color: rgba(20,  122,  188,  0.5);
    background: rgba(20,  122,  188,  0.5);
    color: rgba(20,  122,  188,  0.5);
    padding: 6px 0 12px;
    height: 2px;
    transform:  rotate(358deg) scale(1) skew(0deg) translate(0px);-webkit-transform:  rotate(358deg) scale(1) skew(0deg) translate(0px);-moz-transform: rotate(358deg) scale(1) skew(0deg) translate(0px);-o-transform:  rotate(358deg) scale(1) skew(0deg) translate(0px);-ms-transform:  rotate(358deg) scale(1) skew(0deg) translate(0px);
}

#footer-top3 {
    color: #fff;
    background-color: rgba(128,  185,  46,  0.7);
    background: rgba(128,  185,  46,  0.7);
    color: rgba(128,  185,  46,  0.7);
    transform:  rotate(358deg) scale(1) skew(-2deg) translate(0px);
    -webkit-transform:  rotate(358deg) scale(1) skew(-2deg) translate(0px);
    -moz-transform: rotate(358deg) scale(1) skew(-2deg) translate(0px);
    -o-transform:  rotate(358deg) scale(1) skew(-2deg) translate(0px);
    -ms-transform:  rotate(358deg) scale(1) skew(-2deg) translate(0px);
}

#footer-bottom {
    color: #808080;
    background-color: #2D2D2D;
    font-size: 0.916em;
    padding: 30px 0;
    border-top: 1px solid #000;
    box-shadow: 0 4px 4px rgba(0,0,0,0.2) inset;
    position: relative; /* Added */
}

<div id="footer-top2"></div>
<div id="footer-space"></div>
<div id="footer-top3">
    <div id="footer-top">
    <div class="container clearfix">
        <div class="one-fourth">
                <div class="widget twitter-widget">
                    <h3>Ãšltimos Tweets</h3>
                    <div class="tweet"></div>
                </div>
            </div>
            <div class="one-fourth">
                <div class="widget twitter-widget">
                    <h3> Facebook Feed</h3>
                    <div class="tweet"></div>
                </div>
            </div>

            <div class="one-fourth">
                <div class="widget twitter-widget">
                    <h3>Fotos en Instagram</h3>
                    <div class="tweet"></div>
                </div>
            </div>


            <div class="one-fourth column-last">
                <div class="widget contact-info">
                    <h3>Contacto</h3>
                    <div>address here
                        </div>
                </div>
            </div>
        </div>
    </div>
</div>
<div id="footer-bottom">
    <div class="container clearfix">
    Footer content
    </div>
</div>

Question 2

You could try achieving this with a css gradient. I do something similar on my website's splashpage :

http://agency89ninety.com

The angled background on the above site is purely CSS3 gradients (with fallbacks of course)

This is done by creating a gradient, starting at 0% with a color of GREY, ending at 50% with a color of GREY as well, then starting another gradient at 50% with a color of DARKGREY and ending at 100% with a color of DARKGREY also.

Basically you are tricking the system. It's technically a gradient, but looks like two solid colors that meet in the middle and are angled.

Take a look at this site to learn more about the properties of a css gradient:

http://www.css3files.com/gradient/