Irregular shaped frames/borders in html or css - html

Question

I am fairly new to html and css and I have been tasked to create a layout with irregularly shaped borders as depicted here: frame pic
I removed all the other content. I currently have no clear handle on how to approach the white area with the triangular pointer pointing at the button.
(That shape will contain more content.)
What would you suggest?

Answer 1

One method would be to rotate a square div by 45 degrees and have two borders visible.
Using the snippet provided by #quid:
#arrow {
background: #ffffff;
border-top: 1px solid #dcdcdc;
border-left: 1px solid #dcdcdc;
position: absolute;
height: 30px;
width: 30px;
top: 0;
right: 20px;
margin-top: -16px;
-webkit-transform: rotate(-45deg);
-moz-transform: rotate(-45deg);
-o-transform: rotate(-45deg);
-ms-transform: rotate(-45deg);
transform: rotate(45deg);
}

Answer 2

added the arrow from #Banderi
Here's a fiddle with this working: http://jsfiddle.net/frnm9ymo/7/
With this most recent edit I even matched the colors for you.
It's not the most elegant solution I'm sure, but it works. You just make the arrow image match the colors of the box then overlap the border by whatever the width of the border is. Inside the red box would be a png image with a transparent background. The most important thing to remember is the arrow div will be "absolute" positioned and it's parent div must be "relative" positioned.
CSS:
* {
font-family: arial;
}
#wrapper {
background: #c7dae7;
width: 400px;
nin-height: 400px;
display: block;
float: left;
}
#container {
margin: 20px;
display: block;
background: #f6f7fb;
border: 1px solid #dfe2eb;
}
#titlebar {
margin: 0;
padding: 15px 20px;
display: block;
background: #fff;
overflow: none;
min-height: 30px;
border-bottom: 1px solid #dfe2eb;
}
#contents {
margin: 0 0 -40px 0;
padding: 7px;
display: block;
}
#textwrap {
margin: 5px;
padding: 10px 20px;
display: block;
background: #ffffff;
border: 1px solid #dcdcdc;
position: relative;
-webkit-border-radius: 15px 5px 15px 5px;
-moz-border-radius: 15px 5px 15px 5px;
border-radius: 15px 5px 15px 5px;
}
textwrap p {
z-index: 100;
}
#enter {
float: right;
width: 80px;
padding: 7px;
background: #4992e6;
color: #fff;
font-weight: bold;
border: 0;
-webkit-border-radius: 15px;
-moz-border-radius: 15px;
border-radius: 15px;
}
#arrow {
background: #ffffff;
border-top: 1px solid #dcdcdc;
border-left: 1px solid #dcdcdc;
position: absolute;
height: 30px;
width: 30px;
top: 0;
right: 30px;
margin-top: -17px;
-webkit-transform: rotate(-45deg);
-moz-transform: rotate(-45deg);
-o-transform: rotate(-45deg);
-ms-transform: rotate(-45deg);
transform: rotate(45deg);
z-index: 50;
}
HTML:
<div id="wrapper">
<div id="container">
<div id="titlebar">
text
<button id="enter">
ENTER
</button>
</div>
<div id="contents">
<div id="textwrap">
<div id="arrow">
</div>
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