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.

In Flexbox, the justify-content property is used to define how items are aligned along the main axis. It controls the distribution of space between and around items along the main axis and is commonly used to center items horizontally or vertically.

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.

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 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.

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 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.