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.

To change the font weight, you can use the CSS (Cascading Style Sheets) property font-weight.

In CSS, you can assign different values to the font-weight property to control the thickness of the text. The most commonly used values are:

• normal: This is the default value and represents the normal font weight.
• bold: This makes the text appear thicker and bolder.
• lighter: This makes the text appear thinner and lighter than the normal weight.
• Numeric values: You can also use numeric values between 100 and 900 to specify a specific font weight. For example, 400 is normal and 700 is bold.

Here's an example of how to use the font-weight property in CSS:

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p {
  font-weight: bold;
}

This CSS rule will make all the text within <p> elements appear bold.

You can learn more about the font-weight property in CSS on the Mozilla Developer Network (MDN) documentation: https://developer.mozilla.org/en-US/docs/Web/CSS/font-weight

In computing, "em" is a unit of measurement that is commonly used in web design and typography. It is a relative unit and is typically used to define the size of text or other elements in relation to the font size of the parent element. For example, if the font size of a parent element is 16 pixels, then 1em would be equal to 16 pixels.

Here is an example of how em units can be used in CSS:

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.parent {
  font-size: 16px;
}

.child {
  font-size: 1em; /* This will be 16 pixels */
}

.grandchild {
  font-size: 0.75em; /* This will be 12 pixels (0.75 * 16) */
}

You can learn more about em units in web design here: https://www.w3schools.com/cssref/css_units.asp

To change the letter spacing in CSS, you can use the letter-spacing property. This property allows you to adjust the amount of space between characters in a text element. You can set the letter-spacing value to a specific length, such as pixels or ems, or you can use a percentage to adjust the spacing relative to the font size.

For example, to increase the letter spacing in a paragraph element, you can add the following CSS code:

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p {
  letter-spacing: 2px;
}

This will add a space of 2 pixels between each character in the paragraph. You can also use negative values to decrease the letter spacing if desired.

Alternatively, you can use a percentage value to adjust the letter spacing relative to the font size. For example:

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p {
  letter-spacing: 10%;
}

This will increase the letter spacing by 10% of the font size. You can experiment with different values to achieve the desired visual effect.

Keep in mind that not all fonts and browsers may support the letter-spacing property, so it's always a good idea to test your changes across different browsers and devices to ensure consistent rendering.

To make your font bolder, you can use the font-weight property in CSS. The higher the value of the property, the bolder the font will appear. For example:

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.element {
  font-weight: 700;
}

Here, a font-weight value of 700 is used, which is a commonly used value for bold font. However, note that not all fonts have a bold value of exactly 700, so you may need to try different values to find the one that works best for your font.

The line-height property in CSS is used to control the vertical space between lines of text within an element. It determines the amount of space that is available above and below the text itself. The line-height value can be set in different ways:

• As a numeric value which is multiplied by the font-size of the text, e.g. line-height: 1.5; which means the line height is 150% of the font-size.
• As a length value, e.g. line-height: 20px; which sets the exact line-height.
• As a percentage value, e.g. line-height: 120%; which sets the line height as a percentage of its parent element's font size, but the actual value will depend on the size of the text.

An appropriate line-height can improve readability, make text more visually comfortable to scan through and is important when designing responsive websites.

Here's an example of how to set line-height in CSS:

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p {
  font-size: 16px;
  line-height: 1.5;
}

Font size refers to the measurement of the text, typically in points (pt), pixels (px), or ems (em), whereas font weight refers to the thickness or boldness of the characters.

For example, if you have a paragraph of text with a font size of 16px and a font weight of 400, the characters will be a normal weight and thickness. However, if you increase the font weight to 700, the characters will become bolder and thicker.

Here's an example of changing the font size and weight in CSS:

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p {
  font-size: 16px;
  font-weight: 400; /* normal weight */
}

h1 {
  font-size: 24px;
  font-weight: 700; /* bold weight */
}

You can learn more about font sizes and weights in CSS here: https://www.w3schools.com/cssref/pr_font_weight.asp.

To use CSS to underline a heading, you can use the "text-decoration" property and set it to "underline". Here's an example in CSS:

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h1 {
  text-decoration: underline;
}

This will add an underline to all "h1" elements. You can also target other heading elements like "h2", "h3", etc. by replacing "h1" with the desired heading element.

You can learn more about CSS text-decoration property in here.

<em> is an HTML tag that is used to emphasize text. It represents a section of text that should be displayed in italic or emphasized font. Here's an example of how to use it:

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<p>This <em>text</em> is emphasized.</p>

In this example, the word "text" will be displayed in italics because it is wrapped in the <em> tag.

To make font italic, you can use the CSS font-style property and set its value to italic.

Here's an example in CSS:

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p {
  font-style: italic;
}

In HTML, you can apply this style to a specific element using its class or ID like this:

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<p class="my-italic-text">This text will be italic</p>

<!-- CSS -->
.my-italic-text {
  font-style: italic;
}

To make font italic in HTML, you can use the <em> or <i> tag. Both tags will provide italicized formatting to the enclosed text. Here's an example snippet:

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<p>This is <em>italicized</em> text using the &lt;em&gt; tag.</p>
<p>This is <i>italicized</i> text using the &lt;i&gt; tag.</p>

You can also use CSS to apply an italic font style to a specific CSS class or ID. Here's an example snippet:

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.italic {
  font-style: italic;
}

Then apply the italic class to the HTML element you want to italicize:

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<p class="italic">This paragraph text is italicized via CSS.</p>