The backface-visibility property in CSS is particularly useful when creating 3D transformations. When set to hidden, it prevents the back side of an element from being visible, which is essential for 3D transformations where the front and back of an element should not be visible simultaneously.

To ensure that the distance between lines of text is 1.5 times the size of the text, you can use the line-height property in CSS. You can set it to 150%, which is equivalent to 1.5 times the font size. Alternatively, you can use line-height with an em value, like 1.5em, which achieves the same effect.

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.

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.

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.

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 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.

To create a vintage look for images, you can use CSS filters. The combination sepia(100%) contrast(0.8) brightness(0.8) saturate(1.5) would add a sepia tone, reduce contrast, lower brightness, and slightly increase saturation, giving the image a vintage appearance.

To align the content of a grid item along the block (column) axis, you should use the 'align-content' property. 'align-content' aligns the items within the grid container along the block axis, such as centering them vertically. 'justify-content' and 'align-items' are used for the main and inline axes, respectively. 'align-self' is used to control the alignment of individual grid items.

The CSS property used to specify the number of times an animation should run is animation-iteration-count. This property allows you to control how many times an animation sequence repeats. If set to a specific number or "infinite," it determines the animation's iteration count.

To use the attr() function to get the value of a data attribute in CSS, you should specify the attribute's name as an argument within attr(). For example, to retrieve the value of a data attribute named "data-value," you would use attr(data-value). You can then apply this value to a CSS property, such as content or a custom property.

To target devices with retina displays specifically, you can use the @media query and specify the min-device-pixel-ratio property. The value of 2 represents a typical retina display, where each CSS pixel corresponds to 2 physical device pixels. So, the correct option is @media (min-device-pixel-ratio: 2).