To underline text in CSS, you should use the text-decoration property and set it to "underline." This CSS rule adds a line beneath the text.

The property used to control how inherited values and set values are used by an element and its child elements is the inherited property. It allows for the explicit declaration of whether a property should be inherited by child elements. This is particularly useful for managing the cascading nature of CSS.

The space between the content of an element and its border is determined by the "padding" property in CSS. Padding provides the spacing between the content and the border of an element, allowing you to control the inner spacing.

To ensure that fonts are loaded from the same origin, you can use the Access-Control-Allow-Origin HTTP header. This header specifies which origins are permitted to access a resource, which is useful for preventing cross-origin resource sharing issues.

To set a fallback font in case the primary font fails to load, you can use the font-family property and specify multiple font names in a comma-separated list. The browser will use the first font in the list that it can find and load. This ensures that even if the primary font is unavailable, the text will still be displayed using an alternative font.

To provide a fallback value for a CSS custom property, you can use the var() function. The first argument of var() is the custom property, and the second argument is the fallback value. If the custom property is not defined, the browser will use the fallback value. For example, --custom-property: var(--fallback, fallback-value); sets --custom-property to the value of --fallback if it exists, or "fallback-value" if --fallback is not defined.

When developing a slide-in sidebar menu or any animation, using the transform property is the most efficient choice for smooth performance. This is because the transform property can be hardware-accelerated by the browser, leading to faster and smoother animations. Using properties like margin, padding, or width for animations can cause reflows and repaints, which can be less performant.

To target an input element in CSS based on its validation state, you can use the :invalid pseudo-class. This selector matches input elements that have failed validation, such as a required field left blank or an email input with an invalid format. For example, input:invalid { border: 2px solid red; } can be used to style invalid input elements.

In CSS transitions, the "transition-delay" property is used to introduce a delay before the transition begins. This allows you to control when the transition effect starts after a triggering event, such as a hover or click.

The main advantage of using CSS-in-JS is improved code maintainability. With CSS-in-JS, styles are encapsulated within the component they belong to, reducing the chances of style conflicts and making it easier to manage and maintain styles in a complex application. It promotes a more component-oriented and self-contained approach to styling, enhancing the overall maintainability of your codebase.