2 Answers. It says: "Outline differs from borders! Unlike border, the outline is drawn outside the element's border, and may overlap other content. Also, the outline is NOT a part of the element's dimensions; the element's total width and height is not affected by the width of the outline." When you set an outline it should NOT be affected by The plugin is adding global wild card css that changes all elements in each other plugin and theme to box-sizing: border-box. It is done in the ep-helpers.css and has the following css. *, *::before, *::after {box-sizing:border-box}. The css sets every element as well as each elements before and after pseudo classes (*, *::before Note also that box-sizing: border-box without the -moz-prefix should appear in the given CSS so other browsers will apply the same box sizing as well. The -moz-prefix is used by Firefox up to version 28 (the just-released version 29 ships with unprefixed box-sizing). See this answer. According to w3c specs for Box Model. The rendered width of a box type element is equal to the sum of its width, left/right border and left/right padding. So that means the values of padding and border-width affects the total width of the element. Use box-content to set an element’s box-sizing to content-box, telling the browser to add borders and padding on top of the element’s specified width or height. This means a 100px × 100px element with a 2px border and 4px of padding on all sides will actually be rendered as 112px × 112px, with an internal content area of 100px × 100px content-box is the default, and gives you the default CSS box-sizing behavior. If you set an element's width to 100 pixels, then the element's content box will be 100 pixels wide, and the width of any border or padding will be added to the final rendered width. box-sizing: content-box; uses the box-model that everyone is used to, sometimes called the “W3C Box Model”. While box-sizing: border-box; uses the box-model that people have come to associate with Internet Explorer, where the dimensions of the padding and border are included in the element’s dimensions. Take a moment to look at the Hi, In this video , i have explained what is the difference between content box and border box in css. It is very important to know when you create a web lay HsYt00.