In web design, rem, px, and em are units of measurement used for CSS properties, especially for font sizes and spacing.

• px (pixels): This is a fixed unit. If you set an element's width to 100px, it will always be 100 pixels wide, regardless of the user's settings or screen size. It's precise but not scalable.

• em: This unit is relative to the font size of its parent element. If you set an element's font size to 2em, it will be twice the size of its parent element's font size. For example, if the parent font size is 16px, 2em would equal 32px. This is useful for creating responsive designs.

• rem (root em): Similar to em, but it is relative to the root (html) font size. It provides a more consistent sizing because it doesn't depend on the parent element's size. If the root font size is 16px and you set an element's size to 2rem, it will be 32px.

Here's an example in CSS:

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body {
    font-size: 16px; /* root font size */
}

h1 {
    font-size: 2rem; /* 2 * 16px = 32px */
}

p {
    font-size: 1.5em; /* if parent font size is 20px, then 1.5em = 30px */
}

footer {
    font-size: 14px; /* parent size for example */
}

footer p {
    font-size: 1em; /* 1 * 14px = 14px because this is relative to footer */
}

In this example, the h1 will always be twice the root size, while the p elements will vary depending on their parent size.

em and px are both units of measurement commonly used in web development for font sizes and other page elements.

px, short for "pixel," is a fixed-size unit of measurement that is based on the display resolution of the device. One px is equal to one dot on the screen. This means that the size of a px unit will depend on the screen's pixel density - on a device with a higher pixel density, one pixel will be smaller than on a lower-density device.

em is a relative unit of measurement that is based on the size of the parent element. For font sizes, one em is equal to the font size of the parent element. For example, if a parent element has a font size of 16px, then 1em would be equal to 16px. If a child element has a font size of 0.5em, then its font size would be 50% of the parent element's font size.

Here is an example of how to use em and px units in CSS:

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.parent {
  font-size: 16px;
}

.child {
  font-size: 0.8em; /* 80% of parent font size */
  width: 20px; /* fixed width in pixels */
}

When setting the spacing between elements, the best option to use depends on the context of your project.

• px is a fixed unit of measurement and is not affected by the size of the browser viewport or the base font size defined for the project. This makes it ideal for smaller elements such as buttons.

• rem is a relative unit that is based on the root element’s font-size. It's good to use for larger components like headers, as it adjusts in size in accordance with the base font size.

• em is also a relative unit and is based on the font size of the parent element. It's good for context-specific sizing of elements.

You can also use a combination of these units depending on the different elements in your project to achieve the desired spacing.

Here is an example of using rem for a margin of 20 pixels:

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.my-class {
  margin: 1.25rem; /* assuming the root font-size is 16px */
}

Reference: https://www.w3schools.com/cssref/css_units.asp

When deciding between using em, rem, or px for sizing, it's best to consider the needs of your design and accessibility.

• Use px when you need fixed-size elements that won't change, such as for borders or shadows.
• Use rem for font sizes to make it more accessible to users who have changed their browser's default font size. rem is relative to the root font size or default font size of the browser.
• Use em when you need to set a size relative to its parent, such as for a button that needs to grow or shrink within its container. em is relative to its parent element's font size.

It's important to keep in mind that using relative units like em and rem ensures that your design remains consistent and adaptable to different screen sizes and user preferences. For more information, you can check out this article about responsive typography using rem and em: https://www.smashingmagazine.com/2017/07/enhancing-typographic-hierarchy-with-css-rem-and-em-values/

When it comes to styling elements on a webpage, there are a number of ways to define font size including using em, rem, and px. Here is a brief explanation of how and when to use each unit:

• px (pixels) is an absolute unit and it sets the font size of an element to a specific number of pixels. This unit is often used for small font sizes or specific design requirements, but it can be problematic for accessibility because users may have trouble resizing text on some devices.

Example (CSS):

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p {
  font-size: 16px;
}

• em is a relative unit that is based on the parent element's font size. This is useful for proportional scaling of elements. For example, if a parent element has a font size of 16px and the child element is set to 2em, the font size would be 32px.

Example (CSS):

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.parent {
  font-size: 16px;
}
.child {
  font-size: 2em; /* 32px */
}

• rem is another relative unit that is based on the root element's font size. This allows for consistent scaling across all elements on a webpage, regardless of the context. It can be particularly useful for responsive design.

Example (CSS):

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html {
  font-size: 16px;
}
.element {
  font-size: 1.25rem; /* 20px */
}

Overall, em and rem are more flexible and adaptable than px. It's a good idea to consider the context and requirements of your design when choosing which unit of measure to use.

em and px are units of measurement used to specify the size of elements on a webpage.

px stands for pixels, and it is a fixed-size unit of measurement that does not change with resizing.

em stands for "em"phasis, and it is a relative unit of measurement that adjusts based on the font-size set for a parent element.

For example, if you set the font-size of a parent element to 16px and you set the font-size of a child element to 1em, the child element will have a font size of 16px because 1em is equal to the font-size of the parent element.

css
 
 
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p {
  font-size: 16px;
}

span {
  font-size: 0.8em;
}

In this example, the font size of the span element will be 80% of the font size of the p element because 0.8em is equal to 80% of the font-size of the parent element.

You can learn more about em and px units here.

In CSS, you can create spacing between elements using the margin property. For example:

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p {
  margin: 20px;
}

This will add 20 pixels of space between any <p> elements. For more information, you can check out MDN's reference on the margin property.

Your font size may not be updating since you are using rem units instead of px units. rem units are based on the root element's (most commonly the <html> element) font size; using rem ensures that the element will automatically scale up or down relative to the <html> element. If your font size is not updating, you may need to edit the <html> element's font size. Depending on which stylesheet language you are using, you may need to use:

• CSS: font-size: 16px;
• SCSS: html { font-size: 16px; }
• Less: @html-font-size: 16px;

The difference between PX and EM is that PX is an absolute unit in CSS and EM is relative. PX is an absolute unit of measurement, meaning it will always be the same size regardless of the size of the viewport, while EM is relative to the font size of the parent element. PX is mainly used for fixed-width items such as font size, padding, and margins, while EM is mainly used for items that can have different measurements, such as headings, images, and text. More information can be found here.