The primary difference between clip-path and mask in CSS is that clip-path clips an element to a specified shape, whereas the mask property applies an image as a mask to an element. This fundamental distinction affects how elements are visually modified and displayed.

To create a layout where a sidebar stays to the left and the main content takes up the remaining width, you would float the sidebar to the left and set the main content's width to 100%. This ensures that the sidebar stays to the left while the main content occupies the remaining available space.

In SASS/SCSS, you can conditionally apply styles using if/else statements. These statements allow you to set specific styles based on certain conditions, making your stylesheets more dynamic and responsive to different scenarios.

To style the first line of a paragraph to be bold, you should use the ::first-line pseudo-element. This allows you to target and style the first line of text within an element. ::before and ::first-letter are used for different purposes, and ::first-child targets the first child element, not the first line of text.

The CSS property used to vertically align inline elements or inline-block elements is vertical-align. It allows you to control how the element aligns in relation to the surrounding content or other inline elements. It's commonly used for aligning elements such as images and text within a line of text.

To style list items that are not the first child, you can use the pseudo-class ":not(:first-child)." This allows you to target and style all list items except the first one in a list.

The em unit is relative to the font-size of the element itself, while the rem unit is relative to the font-size of the root element (html). This means that em units can compound if used within nested elements, whereas rem units remain consistent throughout. Understanding this difference is crucial for responsive design.

For ensuring compatibility with the last two versions of major browsers, Autoprefixer is the ideal tool. It automatically adds vendor prefixes to your CSS code, ensuring that your styles work in various browsers. This is essential for maintaining cross-browser compatibility.

PostCSS is a tool used to process CSS and extend it with new features. It allows developers to use future CSS syntax by transforming it into standard CSS. In this context, the answer is "CSS."

In the SMACSS (Scalable and Modular Architecture for CSS) methodology, states like "is-active" or "is-hidden" are typically handled by applying them as classes directly in the HTML markup. This allows for clear separation of styling and behavior, making the code more maintainable and modular.