Cascading Style Sheets (CSS) is a design language that defines the layout of HTML documents and is commonly applied to webpages and browser-based content. Where HTML is used to structure content, CSS allows more precise control of the layout of your content, as well as the fonts, colours, margins, tables and backgrounds within the content.
According to Wikipedia,
"The World Wide Web Consortium (W3C) considers the advantages of CSS for defining all aspects of the presentation of HTML pages to be superior to other methods."
Prior to CSS, HTML documents were complex and difficult to maintain, with web developers having to mark up each element of presentation literally line by line. CSS was developed to accommodate a variety of styles and provide greater control of how content was presented, while producing simpler HTML.
The term cascading refers to how the style sheet operates. The style sheet with the highest priority controls how the content is displayed but if these styles are not set, then presentation is passed on to a source of lower priority such as the browser's default settings for presenting design elements.
Inline styles in HTML take highest priority and will render your content accordingly, which is what the system’s CSS Inline converter tool does: automatically setting your styles to the highest priority, line by line, without you having to edit the HTML.
In most instances, webmail clients like Gmail, Yahoo or Hotmail strip any CSS formatting or style sheets due to conflicts with their own HTML tags and that’s the reason an email looks different in Gmail compared to MS Outlook.
If you send out an email campaign and your CSS is not coded inline, it's likely to get stripped out of online email clients, resulting in shoddy-looking email design. The CSS inline converter takes away the worry of emails not rendering correctly, allowing your emails to be rendered more accurately and beautifully, without you having to worry about coding.