Question 1

CSS Pixel (px):

Accepted Answer

Often misunderstood as a hardware dot, a CSS pixel is actually an angular measurement of resolution. The W3C defines a CSS pixel as exactly 1/96th of an inch. On a standard 96 DPI screen, one CSS pixel equals one device pixel. On a high-density Retina screen with a Device Pixel Ratio (DPR) of 2 or 3, a single CSS pixel spans 4 or 9 physical hardware pixels, respectively.

Question 2

Root Element:

Accepted Answer

In HTML documents, the root element is universally the <html> tag. It is the absolute highest level of the Document Object Model (DOM) tree. When you apply styling to the :root pseudo-class or the html selector in CSS, you are establishing baseline rules that cascade down to every other element on the page.

Question 3

Default Browser Font Size:

Accepted Answer

Every web browser (Chrome, Safari, Firefox, Edge) contains an internal stylesheet that dictates how unstyled HTML should look. For over two decades, the universal default font size in these internal stylesheets has been 16px. This is the foundation upon which all rem math is based unless explicitly overridden by the user or the developer.

Question 4

Compounding:

Accepted Answer

This is the mathematical snowball effect that occurs when using standard em units. Because em is relative to its parent, nested elements multiply their values. If a parent is 1.5em and a child is 1.5em, the child's actual rendered size is 2.25em relative to the root. The rem unit was specifically invented to bypass compounding by always referencing the root element.

Question 5

Web Content Accessibility Guidelines (WCAG):

Accepted Answer

The internationally recognized set of standards for web accessibility published by the W3C. WCAG Success Criterion 1.4.4 specifically mandates that text must be resizable up to 200% without the use of assistive technology and without breaking the layout. Using rem is the most effective technical strategy for meeting this legal and ethical benchmark.

Question 6

REM vs. PX:

Accepted Answer

The defining battle of CSS units. Pixels offer absolute, uncompromising control. If you set a box to 500px, it will be exactly 500 CSS pixels wide, regardless of user settings. This is ideal for fixed media, canvas elements, or strict ad banners. However, pixels fail completely at accessibility. REM units surrender absolute control to the user's browser settings, prioritizing accessibility and fluid scaling over rigid, pixel-perfect design. For almost all UI text and layout containers, REM is objectively superior.

Question 7

REM vs. EM:

Accepted Answer

Both are relative units, but they relate to different anchors. rem is strictly tied to the root <html> element, providing a consistent, predictable size anywhere on the page. 1rem is the same size in the header, the footer, and a deeply nested sidebar. em, however, is relative to its immediate parent element. This makes em incredibly powerful for modular components (like a scalable button), but extremely dangerous for global layout due to the compounding effect where nested elements multiply their sizes exponentially.

Question 8

REM vs. VW / VH:

Accepted Answer

Viewport Width (vw) and Viewport Height (vh) are relative to the physical dimensions of the browser window. 1vw is exactly 1% of the screen width. While viewport units are excellent for creating full-screen hero sections or fluid typography, they completely ignore the user's default font size settings. If a user with bad vision is on a small mobile phone, 5vw will render as tiny text, and the user cannot scale it up via browser settings. Therefore, vw should rarely be used in isolation for typography; it should be combined with rem using the clamp() function.

Question 9

REM vs. CH:

Accepted Answer

The ch unit represents the width of the character "0" (zero) in the current font. It is highly specialized and incredibly useful for setting optimal line lengths for reading. Typographic standards suggest a line length of 60 to 75 characters. Setting a paragraph container to max-width: 65ch; ensures perfect readability regardless of the font size. While rem is best for vertical sizing and general layout, ch is the superior alternative for horizontal text constraints.

Question 10

Why can't I just use pixels for everything and let the user zoom in with their browser?

Accepted Answer

Browser zooming (using Ctrl/Cmd + Plus) scales the entire page, including images and layouts, which often forces horizontal scrolling on smaller screens. Horizontal scrolling is a massive usability failure and a violation of WCAG standards. Changing the default font size, however, relies on relative units to reflow the text naturally within the layout. If you use pixels, the text refuses to reflow, breaking the accessibility feature entirely.

Question 11

Is the 62.5% CSS trick still recommended by professionals today?

Accepted Answer

It is highly debated. While it makes mental math easier by setting the base to 10px (1.6rem = 16px), many modern developers argue it is an outdated relic from the era before CSS preprocessors. Today, with tools like Sass, PostCSS, or native CSS calc(), the math can be automated. Furthermore, modern frameworks like Tailwind use the standard 16px base, making the 62.5% trick incompatible with much of the current frontend ecosystem.

Question 12

How do rem units affect media queries and responsive breakpoints?

Accepted Answer

Using rem in media queries (e.g., @media (min-width: 40rem)) is highly recommended. If a user increases their browser font size, a pixel-based media query remains static, potentially serving a cramped desktop layout to a user who needs massive text. A rem-based media query scales with the user's font size, meaning the layout will switch to a more spacious mobile or tablet view sooner, accommodating the larger text seamlessly.

Question 13

Do search engines like Google care whether I use rem or px?

Accepted Answer

Search engines do not directly penalize the use of px or reward the use of rem in their core ranking algorithms. However, Google heavily factors Core Web Vitals and mobile usability into its rankings. Because rem units naturally facilitate better responsive design and prevent text-overflow issues on mobile devices, using them indirectly improves your SEO by ensuring a flawless, accessible user experience that passes Google's mobile-friendly tests.

Question 14

How should I handle CSS borders, box-shadows, and border-radii?

Accepted Answer

Borders should almost always be written in px. A 1px border ensures a crisp, single-pixel line. Converting it to 0.0625rem risks the browser rounding it to zero on low-resolution screens. Box-shadows usually look best in px to maintain their precise aesthetic spread, though rem can be used for massive, structural shadows. Border-radius can be either; px ensures a perfectly consistent curve, while rem allows the curve to soften and expand as the element grows.

Question 15

What happens if a user sets their default browser font size smaller than 16px?

Accepted Answer

The exact same mathematical conversion occurs, just in reverse. If a user has perfect vision and wants more data on their screen, they might set their base size to 12px. A heading coded at 2rem (intended to be 32px) will instantly scale down to 24 physical pixels (2 * 12). The entire interface will shrink proportionally, allowing the user to view a denser, more compact version of your application without any layout elements overlapping or breaking.

PX to REM Converter

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