Replicate image in CSS and HTML only - html

Question

For a code quiz I am trying to replicate the attached image using CSS and HTML only. The gradients and borders seem to be working out fine, but I am having trouble getting the ribbon edges to look curved.
Here is my markup:
<div class="container">
<div class="ribbon-wrapper-green"><div class="ribbon-green">AWESOME!</div></div>
<div class="ribbon-bottom-left"></div>
<div class="bottom-left-gradient"></div>
And here is my CSS:
.container {
width:300px;
height:300px;
margin-left:auto;
margin-right:auto;
margin-top:10px;
position:relative;
border:#CCC 1px solid;
background-color:#EBEBEB;
box-shadow:1px 1px 1px #CCC;
}
.ribbon-wrapper-green {
width: 290px;
height: 290px;
overflow: hidden;
position: absolute;
top: -3px;
right: -3px;
}
.ribbon-green {
font: 25px Arial;
text-align: center;
text-shadow:0px 0px 6px #000;
-webkit-transform: rotate(45deg);
-moz-transform: rotate(45deg);
-ms-transform: rotate(45deg);
-o-transform: rotate(45deg);
position: relative;
padding: 20px 0;
left: 7px;
top: 66px;
width: 363px;
background-color: #BFDC7A;
background-image: -webkit-gradient(linear, left top, left bottom, from(#BFDC7A), to(#8EBF45));
background-image: -webkit-linear-gradient(top, #BFDC7A, #8EBF45);
background-image: -moz-linear-gradient(top, #BFDC7A, #8EBF45);
background-image: -ms-linear-gradient(top, #BFDC7A, #8EBF45);
background-image: -o-linear-gradient(top, #BFDC7A, #8EBF45);
color: #FFF;
-webkit-box-shadow: 0px 8px 20px rgba(0,0,0,0.3);
-moz-box-shadow: 0px 8px 20px rgba(0,0,0,0.3);
box-shadow: 0px 8px 20px rgba(0,0,0,0.3);
}
.ribbon-green:before, .ribbon-green:after {
content: "";
border-top: 4px solid #6e8900;
border-left: 4px solid transparent;
border-right: 4px solid transparent;
position:absolute;
bottom: -4px;
}
.ribbon-green:before {
left: 0;
}
.ribbon-green:after {
right: 0;
}
.ribbon-bottom-left {
width: 0px;
height: 0px;
border-style: solid;
border-width: 250px 0 0 250px;
border-color: transparent transparent transparent #D32023;
position:absolute;
bottom:-5px;
left:-5px;
border-top-left-radius:10px;
border-bottom-right-radius:10px;
-moz-border-radius-topleft:10px;
-moz-border-radius-bottomright:10px;
z-index:1;
}
.bottom-left-gradient {
position:absolute;
bottom:-8px;
left:-8px;
z-index:100;
height:245px;
width:245px;
opacity:0.4;
filter:alpha(opacity=40);
background: -moz-linear-gradient(45deg, rgba(255,255,255,1) 0%, rgba(255,255,255,0.92) 4%, rgba(255,255,255,0) 51%); /* FF3.6+ */
background: -webkit-gradient(linear, left bottom, right top, color-stop(0%,rgba(255,255,255,1)), color-stop(4%,rgba(255,255,255,0.92)), color-stop(51%,rgba(255,255,255,0))); /* Chrome,Safari4+ */
background: -webkit-linear-gradient(45deg, rgba(255,255,255,1) 0%,rgba(255,255,255,0.92) 4%,rgba(255,255,255,0) 51%); /* Chrome10+,Safari5.1+ */
background: -o-linear-gradient(45deg, rgba(255,255,255,1) 0%,rgba(255,255,255,0.92) 4%,rgba(255,255,255,0) 51%); /* Opera 11.10+ */
background: -ms-linear-gradient(45deg, rgba(255,255,255,1) 0%,rgba(255,255,255,0.92) 4%,rgba(255,255,255,0) 51%); /* IE10+ */
background: linear-gradient(45deg, rgba(255,255,255,1) 0%,rgba(255,255,255,0.92) 4%,rgba(255,255,255,0) 51%); /* W3C */
filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#ffffff', endColorstr='#00ffffff',GradientType=1 ); /* IE6-9 fallback on horizontal gradient */
border-style:solid;
border-width:0 0 5px 5px;
border-radius:10px;
-moz-border-radius:10px;
border-color:#E41316;
}
The image is coming out like this example here: http://arunsood.me/code-quizzes/placebo-effect/.
The original I need to copy is below. How can I achieve the edge bending effect via CSS alone?