How to Make Text Bold in CSS: An In-Depth Technical Guide

Bolding text is used extensively in web design for grabbing attention and highlighting important information. Making text bold with CSS is simple on the surface, but there are technical considerations for text rendering and optimization under the hood that we will cover in this comprehensive guide.

We will start with the basics and progressively get more advanced:

• Font weight basics
• Setting bold text
• Controlling degree of boldness
• Partial text formatting
• Browser support details
• Performance optimization
• Dynamically adjusting weight
• Technical issues to be aware of

Whether you are a beginner looking to make bold text for the first time or an experienced developer wanting to better understand CSS font weights at a lower level, this guide has you covered. Time to dive in!

Font Weight Basics: How Text Gets Bold

Before looking specifically at CSS, let‘s briefly explain what happens behind the scenes to make digital text appear bold.

At the most basic level, bold text is just text that has increased thickness and intensity. This helps it stand out better from regular text on a webpage or document.

In physical printing presses, this effect is created by overprinting letters to make them darker and thicker. But how do we achieve the same impact with website text?

Use of Multiple Font Files

On the web, the bold appearance is accomplished using font files that contain different styles – normal, bold, italic, etc. When you apply a bold CSS style, it switches the displayed font file to the one with the heavier weight.

So a font family like Arial would have separate Arial normal, Arial bold, and Arial black font files included. The CSS font property just points to different files as needed.

As a developer, we don‘t need to manipulate the font files directly. Just understand that bold text comes from loading an alternate style variant behind the scenes.

Now let‘s see how we control which variant gets loaded in.

Setting Text to Bold with CSS

The CSS property that gives us access to these font style variants is font-weight. As we set this property, the browser handles switching the necessary font file to match.

Here is a quick example:

/* Load normal font file */
p {
  font-weight: 400; 
}

/* Switch to bold file */
strong {
   font-weight: bold;
}

When the paragraph text uses a font-weight of 400, it displays the regular font file. But where we apply the bold property, it will swap to the bold font variant instead to make that text thicker.

This is an abstraction on top of what is happening under the hood. But as developers, all we need to do is adjust the CSS weight values to control boldness.

In the next sections we will explore the font-weight property more including how to fine tune the degree of boldness.

Degree of Boldness Values

As mentioned earlier, font files contain different weights – from thin to black. The font-weight property allows access to these weights.

There are two types of values that can be used:

1. Keyword values – bold, bolder, lighter
2. Numeric values – 100, 400, 700

Let‘s look closer at each format…

Boldness Keywords

The keyword values provide some simple relative options for adjusting from the parent weight:

font-weight: normal; /* 400 */ 
font-weight: bold; /* 700 */
font-weight: bolder; /* Next heavier weight */
font-weight: lighter; /* Next lighter weight */

We won‘t focus on normal or lighter here. The ones relevant to bolding text are bold and bolder.

• bold – Jumps to a standard bold weight of 700
• bolder – Goes one weight higher than parent element

Bold will always equate to 700, while bolder is relative. The exact numeric weight of bolder depends on context:

h2 {
   font-weight: 500; /* Medium */
}

h1 {
   font-weight: bolder; /* 800 Extra bold */
} 

p {
   font-weight: bolder; /* 700 Bold */ 
}

So bold explicitly sets 700 weight whereas bolder increases from parent. Either can work depending on the situation.

Numeric Font Weights

The other method is specifying an exact weight number like:

font-weight: 400;
font-weight: 600; 
font-weight: 900;

Numeric weights allow you to jump to exact values instead of relative control.

Here is a mapping of numeric values to common terms:

• 100-300 – Thin/Ultra-light/Light
• 400 – Regular/Normal
• 500 – Medium
• 600-700 – Semi-bold/Bold
• 800-900 – Extra-bold/Black

Some things to note:

• 400 is the default normal appearance.
• 700 is the standard bold.
• 900 is extra or ultra bold.

There can be some variation across fonts, but these general ranges as a good starting point.

Using numbers allows you to precisely control degree and granularity between the presets of normal and bold if needed:

font-weight: 550; /* Midway between medium and bold*/
font-weight: 750; /* Bolder than 700 but not quite 900 */

This fine grain control can be useful in some cases. But be aware that availability of weights depends on the font family itself. Not all fonts contain every 100 value variant.

Now that we understand how to control boldness itself, let‘s look at applying it…

Applying Bold Formatting

Bolding all text in an element like <p> or <h1> is straightforward:

p {
  font-weight: bold; 
}

But there are also easy ways to selectively format just a portion of text to bold:

Here is some <b>selectively bolded</b> text 

HTML Options:

• <b> – Semantic bold tag
• <strong> – Indicates importance
• <span> – Generic wrapper

CSS Options:

• Target child element
• Use pseudo-element
• Apply class

Formatting only some words or phrases bold gives greater flexibility when bolding text. This covers the standard ways to bold parts rather than entire text blocks.

Browser Support and Reliability

A common question around CSS techniques is what browser support and reliability is like. Let‘s analyze browser handling of the font-weight property and bold text specifically…

Overall Browser Support

Desktop support for standard bold text and numeric font weights is excellent at this point:

(source: caniuse.com)

Over 95% global coverage across modern browsers makes numeric weights very reliable.

The keyword values have even better support – bold works across ALL browsers and platforms, while bolder/lighter have good coverage still.

So all standard methods we covered should work well. Always test across browsers, but getting basic bold text consistently should not be an issue at this point.

Potential Reliability Issues

The main compatibility issues that can pop up occasionally:

1. Specific heavier font weights – 900/950 Black may not render perfectly in less common fonts across browsers.

2. Variable fonts – Still need broader support in browsers and apps. Can sometimes cause sporadic issues with bold formatting.

For these reasons best practice is:

• Stick to standard 400 and 700 weights when possible.
• Avoid pushing to extreme ends of the spectrum when reliability is critical.

Testing across browsers helps identify any quirks like with specific font families. But formal bold text presents minimally issues – it‘s well supported across all devices and browsers.

Now that we covered the display side, let‘s talk about optimizing loading performance…

Page Weight and Loading Performance

A key consideration with using multiple font formats is avoiding unnecessary bloating of page file size. Every font variant included increases bandwidth.

We want text bold when needed but inefficient loading directly hurts user experience on websites.

Let‘s analyze optimization strategies around font weights focusing on impact to user…

@font-face {
  font-family: MainFont;
  src: url(fonts/MainFont-Regular.woff2); 
  font-weight: 400;  
}

@font-face {
   font-family: MainFont;
   src: url(fonts/MainFont-Bold.woff2);
   font-weight: 700;     
}

This set up loads both regular and bold font files. What are better options?

Lazy Load Non-Critical Variants

Loading every weight upfront is often unnecessary. Defer less critical variants to be fetched only as needed:

/* Loaded right away */
src: url(MainFont-Regular.woff2); 

/* Loaded later if bold text needed */ 
src: url(MainFont-Bold.woff2);

This helps minimize file requests down to most essentials first.

Combine Weights into Variable Font

Variable fonts consolidate all weights into one font file. Cleaner request logic:

/* All weights in one file */
src: url(MainFont-Variable.woff2);

Reduces overall page weight since only one font asset needed.

Watch Local Storage Caching

Browser caching can also alleviate impact after initial load. If assets are cached locally, bold text usage won‘t trigger extra network requests.

So optimizing for bold text is about…

• Only loading critical font weights
• Consolidating to variable fonts
• Leveraging local browser caching

This reduces page bloat and keeps loading speed fast even when adjusting text to bold. Optimization best practices make usage very efficient.

Dynamic Control with JavaScript

An additional option beyond basic CSS is manipulating boldness dynamically with JavaScript.

You can directly access element fonts and adjust format based on any conditions:

// Get paragraph element
const p = document.querySelector(‘p‘);

// Make it bold
p.style.fontWeight = ‘bold‘;  

// Listen for resize to toggle on/off
window.addEventListener(‘resize‘, function() {

  if (window.innerWidth > 500) {
    p.style.fontWeight = ‘bold‘;

  } else {
    p.style.fontWeight = ‘normal‘;
  }

})

This allows changing boldness in response to any event – user input, device orientation change etc.

Some examples of dynamic control:

• Bold used text search matches
• Emphasize certain page sections conditionally
• Animate font weights
• Swap heavy fonts on laggy devices

JavaScript opens up many possibilities for adaptive text formatting to optimize user experience.

While basic CSS covers most typical bold text usage, JavaScript dynamic control enables building highly advanced and customized formatting.

Technical Challenges to be Aware Of

We covered a lot about using CSS font-weight to bold text. To wrap up, I want to mention some technical analysis around potential issues you may come across.

Bold Text Not Showing At All

If bold text isn‘t showing no matter the weight used, there are a couple possibilities:

1. Font family doesn‘t have bold variant

Not all fonts contain bold files. If one is missing that font can‘t display bold.

2. Bold font not actually heavier

Sometimes font creators label files incorrectly. A "bold" file may not actually appear bolder.

If bold text doesn‘t display right in certain fonts, double check what weights are available and appearing heavier visually.

Bold Text Appearing Too Light

Opposite issue is when bolded text appears only slightly thicker compared to regular. Reasons this could happen:

1. Lack of heavier font weights

As discussed earlier, font availability past standard 400 and 700 can be limited.

2. Improper OS/Browser rendering

Buggy text rendering on certain systems fails to show stark contrast between weighted fonts.

In these cases, you may be limited in just how bold text can appear with that font or browser. Testing across devices is helpful for catching rendering quirks.

Variable Font Problems

Variable font usage is still evolving so hiccups in implementation can translate to formatting glitches:

• Unexpected fallback font behavior
• Browser compatibility gaps
• Visual bugs toggling weights dynamically

As variable fonts mature these issues should improve. But currently something to watch out for.

Bold Text Style Inconsistencies

Finally, more stylistic issues may arise like:

• X-height changes between weights
• Width/spacing differences
• Dynamic weight shifts causing layout movement

Watch for inconsistent styling compared to static mockups when text reflows going bold.

So those are some technical analysis and pitfalls with bold text in CSS. Now you know what to watch out for!

Conclusion

This guide took you under the hood of using CSS to make text bold, covering both essential basics and technical considerations:

• Font-weight controls which font file variant displays
• Set to 700 standard bold or use relative bolder increase
• Numbers allow precise weights tuning
• Format partial text with HTML tags or CSS
• High support across browsers
• Optimize loading using best practices
• Dynamically change boldness with JavaScript
• Expect rare issues with variable fonts and extreme weights

The key points to take away are:

• Use font-weight for easy text bolding
• Stick to standard 400/700 values when reliability is critical
• Implement performance strategies
• Adjust weights dynamically where needed

Text bolding is one the most common formatting needs on websites to draw attention. Use this comprehensive CSS guide to make bold text with reliability and efficiency.

Now you have all the technical knowledge to use bold text effectively!