What is a user agent stylesheet?

I’m working on a web page in Google Chrome. It displays correctly with the following styles.

table {
    display: table;
    border-collapse: separate;
    border-spacing: 2px;
    border-color: gray;
}

It is important to note that I didn’t define these styles. In Chrome developer tools, it says user agent stylesheet in place of the CSS file name.

Now if I submit a form and some validation error occurs, I get the following stylesheet:

table {
    white-space: normal;
    line-height: normal;
    font-weight: normal;
    font-size: medium;
    font-variant: normal;
    font-style: normal;
    color: -webkit-text;
    text-align: -webkit-auto;
}

table {
    display: table;
    border-collapse: separate;
    border-spacing: 2px;
    border-color: gray;
}

The font-size from these new styles is disturbing my design. Is there a way to force my stylesheets and if possible, completely overwrite Chrome’s default stylesheet?

If <!DOCTYPE> is missing in your HTML content you may experience that the browser gives preference to the “user agent stylesheet” over your custom stylesheet. Adding the doctype fixes this.