The "+" (Adjacent Sibling) combinator in CSS targets elements that are placed immediately after a specific element. It selects elements that share the same parent and are placed directly after the specified element.

In CSS, the "vertical-align" property is used to control the vertical alignment of inline or inline-block elements within a line box. When set to "baseline," it aligns elements to the baseline of their parent element, which is typically the bottom of the characters within the text.

To create a shadow effect behind an element in CSS, you'd use the drop-shadow filter. This filter adds a shadow to an element, allowing you to control its position, color, and blur.

Media queries in CSS are primarily used for creating responsive web design. They allow web developers to adapt the layout and styling of a webpage based on the characteristics of the device, such as screen width, height, and orientation. This is essential for ensuring a consistent and user-friendly experience across various devices, from desktops to mobile phones.

To make an animation alternate between running forwards and then backwards, you would use the value alternate for the animation-direction property. This creates a back-and-forth effect for the animation, making it appear as if it's reversing after each cycle.

When you set the child element's font-size to 1.5em, it multiplies the parent element's font-size. In this case, 1.5em x 20px = 30px. So, the computed font-size of the child element will be 30px.

The CSS property that defines the number of times an animation should run is animation-iteration-count. By specifying a value, such as an integer or "infinite," you can control how many times the animation repeats. This property is essential for creating looping animations with a specific number of iterations or an indefinite loop.

To hide an element but maintain its space in the layout, you would use 'display: none;'. This property effectively removes the element from the display, but it still occupies space, which can be useful for various purposes.

To emphasize the first line of every paragraph differently from the rest of the content, you should use the ::first-line pseudo-element in CSS. This pseudo-element targets the first line of a block-level element and allows you to apply specific styling to it.

When working on a large-scale project, the BEM (Block Element Modifier) methodology is highly appropriate. BEM encourages the creation of modular and reusable CSS code by structuring classes in a way that makes it easy to understand their purpose. It follows a strict naming convention, which helps maintainability as the project grows.