Botão desta forma com CSS

Olá.

Eu realmente não sei te dizer se é possível, mas faça o seguinte: defina o background com o código abaixo e depois diga se deu certo, ok?

Abraços,

Thiago Jourdan.

Código CSS:

background: "data:image/png;base64,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";
Ps.: Não é piada, ele é grande mesmo. Copie e cole esse código e faça o teste.

Amigo, dá sim! Mas precisa usar 2 divs:

<style>

#button {
width:100px;
height:76px;
background:#000;
border-top-left-radius:40px;
border-bottom-left-radius:40px;
}
#button2 {
width:100px;
height:100px;
background:#fff;
border-radius:50%;
margin-left:80px;
margin-top:-12px;
position:absolute;
}
</style>

<div id="button">

<div id="button2"></div>

</div>

O que o Thiago Jourdan postou está errado pois não define a largura nem a altura da div nem nada...

Legal, obrigado aos dois, vou analisar e volto a postar;

Tente isso:

div.botao {

	position: relative;
	float: left;

	width: 200px;
	height: 50px;

	background: #7d7e7d;
	background: -moz-linear-gradient(top, #7d7e7d 0%, #0e0e0e 100%);
	background: -webkit-gradient(linear, left top, left bottom, color-stop(0%,#7d7e7d), color-stop(100%,#0e0e0e));
	background: -webkit-linear-gradient(top, #7d7e7d 0%,#0e0e0e 100%);
	background: -o-linear-gradient(top, #7d7e7d 0%,#0e0e0e 100%);
	background: -ms-linear-gradient(top, #7d7e7d 0%,#0e0e0e 100%);
	background: linear-gradient(to bottom, #7d7e7d 0%,#0e0e0e 100%);
	filter: progid:DXImageTransform.Microsoft.gradient( startColorstr='#7d7e7d', endColorstr='#0e0e0e',GradientType=0 );

	border: 0;

	border-top-left-radius: 40px;
	border-bottom-left-radius: 40px;

	overflow: hidden;
}

div.botao:after {

	content: '';
	position: absolute;

	top: 0;
	right: -15px;

	width: 30px;
	height: 100%;

	background: #FFF;

	border-radius: 100%;

	z-index: 1;
}