A design process based on positioning elements relative to a grid.
Due to the busy nature of people and their reliance on technology, there has been a concerted effort in improving the UX (User Experiences) by ensuring that web content can be viewed/accessed across multiple platforms - irrespective of device, browser. There's a push for code to be adaptable/responsive and this has evoked a movement towards standardization.
This concept represents a hierarchical language used to define HTML structure. Commonly used tags/elements like img, p, h1 give meaningful instructions which are easily understood by the machine. In its purest state - HTML should not contain styling, rather styles are added through specificity via CSS.