Browsers come with a big set of default styles to make things look pretty if the styles are not set by the website. For example Firefox has the following styles defined for the default submit button:
input[type="submit"] {
-moz-appearance: button;
-moz-binding: none;
-moz-box-sizing: border-box;
-moz-user-select: none;
background-color: buttonface;
border: 2px outset buttonface;
color: buttontext;
cursor: default;
font: -moz-button;
line-height: normal;
padding: 0 6px;
text-align: center;
text-shadow: none;
white-space: pre;
}