CSS Interview Questions 1 Flashcards
What is CSS selector specificity and how does it work?
The browser determines what styles to show on an element depending on the specificity of CSS rules. We assume that the browser has already determined the rules that match a particular element. Among the matching rules, the specificity, four comma-separate values, a, b, c, d are calculated for each rule based on the following:
a is whether inline styles are being used. If the property declaration is an inline style on the element, a is 1, else 0.
b is the number of ID selectors.
c is the number of classes, attributes and pseudo-classes selectors.
d is the number of tags and pseudo-elements selectors.
The resulting specificity is not a score, but a matrix of values that can be compared column by column. When comparing selectors to determine which has the highest specificity, look from left to right, and compare the highest value in each column. So a value in column b will override values in columns c and d, no matter what they might be. As such, specificity of 0,1,0,0 would be greater than one of 0,0,10,10.
In the cases of equal specificity: the latest rule is the one that counts. If you have written the same rule into your stylesheet (regardless of internal or external) twice, then the lower rule in your style sheet is closer to the element to be styled, it is deemed to be more specific and therefore will be applied.
I would write CSS rules with low specificity so that they can be easily overridden if necessary. When writing CSS UI component library code, it is important that they have low specificities so that users of the library can override them without using too complicated CSS rules just for the sake of increasing specificity or resorting to !important.
What’s the difference between “resetting” and “normalizing” CSS? Which would you choose, and why?
Resetting - Resetting is meant to strip all default browser styling on elements. For e.g. margins, paddings, font-sizes of all elements are reset to be the same. You will have to redeclare styling for common typographic elements.
Normalizing - Normalizing preserves useful default styles rather than “unstyling” everything. It also corrects bugs for common browser dependencies.
Describe floats (CSS) and how they work.
Float is a CSS positioning property. Floated elements remain a part of the flow of the page, and will affect the positioning of other elements (e.g. text will flow around floated elements), unlike position: absolute elements, which are removed from the flow of the page.
The CSS clear property can be used to be positioned below left/right/both floated elements.
If a parent element contains nothing but floated elements, its height will be collapsed to nothing. It can be fixed by clearing the float after the floated elements in the container but before the close of the container.
The .clearfix hack uses a clever CSS pseudo selector (::after) to clear floats. Rather than setting the overflow on the parent, you apply an additional class clearfix to it. Then apply this CSS:
.clearfix::after {
content: ‘ ‘;
visibility: hidden;
display: block;
height: 0;
clear: both;
}
Alternatively, give overflow: auto or overflow: hidden property to the parent element which will establish a new block formatting context inside the children and it will expand to contain its children.
Describe z-index and how stacking context is formed.
The z-index property in CSS controls the vertical stacking order of elements that overlap. z-index only affects elements that have a position value which is not static.
Without any z-index value, elements stack in the order that they appear in the DOM (the lowest one down at the same hierarchy level appears on top). Elements with non-static positioning (and their children) will always appear on top of elements with default static positioning, regardless of HTML hierarchy.
A stacking context is an element that contains a set of layers. Within a local stacking context, the z-index values of its children are set relative to that element rather than to the document root. Layers outside of that context — i.e. sibling elements of a local stacking context — can’t sit between layers within it. If an element B sits on top of element A, a child element of element A, element C, can never be higher than element B even if element C has a higher z-index than element B.
Each stacking context is self-contained - after the element’s contents are stacked, the whole element is considered in the stacking order of the parent stacking context. A handful of CSS properties trigger a new stacking context, such as opacity less than 1, filter that is not none, and transform that is notnone.
Describe Block Formatting Context (BFC) and how it works.
A Block Formatting Context (BFC) is part of the visual CSS rendering of a web page in which block boxes are laid out. Floats, absolutely positioned elements, inline-blocks, table-cells, table-captions, and elements with overflow other than visible (except when that value has been propagated to the viewport) establish new block formatting contexts.
Knowing how to establish a block formatting context is important, because without doing so, the containing box will not contain floated children. This is similar to collapsing margins, but more insidious as you will find entire boxes collapsing in odd ways.
A BFC is an HTML box that satisfies at least one of the following conditions:
The value of float is not none.
The value of position is neither static nor relative.
The value of display is table-cell, table-caption, inline-block, flex, or inline-flex, grid, or inline-grid.
The value of overflow is not visible.
In a BFC, each box’s left outer edge touches the left edge of the containing block (for right-to-left formatting, right edges touch).
Vertical margins between adjacent block-level boxes in a BFC collapse. Read more on collapsing margins.
What are the various clearing techniques and which is appropriate for what context?
Empty div method - <div style="clear:both;"></div>.
Clearfix method - Refer to the .clearfix class above.
overflow: auto or overflow: hidden method - Parent will establish a new block formatting context and expand to contains its floated children.
In large projects, I would write a utility .clearfix class and use them in places where I need it. overflow: hidden might clip children if the children is taller than the parent and is not very ideal.
Explain CSS sprites, and how you would implement them on a page or site.
CSS sprites combine multiple images into one single larger image. It is a commonly-used technique for icons (Gmail uses it). How to implement it:
Use a sprite generator that packs multiple images into one and generate the appropriate CSS for it.
Each image would have a corresponding CSS class with background-image, background-position and background-size properties defined.
To use that image, add the corresponding class to your element.
Advantages:
Reduce the number of HTTP requests for multiple images (only one single request is required per spritesheet). But with HTTP2, loading multiple images is no longer much of an issue.
Advance downloading of assets that won’t be downloaded until needed, such as images that only appear upon :hover pseudo-states. Blinking wouldn’t be seen.
How do you serve your pages for feature-constrained browsers? What techniques/processes do you use?
Graceful degradation - The practice of building an application for modern browsers while ensuring it remains functional in older browsers.
Progressive enhancement - The practice of building an application for a base level of user experience, but adding functional enhancements when a browser supports it.
Use caniuse.com to check for feature support.
Autoprefixer for automatic vendor prefix insertion.
What are the different ways to visually hide content (and make it available only for screen readers)?
These techniques are related to accessibility (a11y).
width: 0; height: 0. Make the element not take up any space on the screen at all, resulting in not showing it.
position: absolute; left: -99999px. Position it outside of the screen.
text-indent: -9999px. This only works on text within the block elements. This is a widely used and famous trick, but it comes with some downsides like causing performance issues, so you might want to consider using text-indent: 100% instead.
Meta tags. For example by using Schema.org, RDF, and JSON-LD.
WAI-ARIA. A W3C technical specification that specifies how to increase the accessibility of web pages.
Even if WAI-ARIA is the ideal solution, I would go with the absolute positioning approach, as it has the least caveats, works for most elements and it’s an easy technique.
Have you used or implemented media queries or mobile-specific layouts/CSS?
Yes. An example would be transforming a stacked pill navigation into a fixed-bottom tab navigation beyond a certain breakpoint.
Are you familiar with styling SVG?
Yes, there are several ways to color shapes (including specifying attributes on the object) using inline CSS, an embedded CSS section, or an external CSS file. Most SVG you’ll find around the web use inline CSS, but there are advantages and disadvantages associated with each type.
Basic coloring can be done by setting two attributes on the node: fill and stroke. fill sets the color inside the object and stroke sets the color of the line drawn around the object. You can use the same CSS color naming schemes that you use in HTML, whether that’s color names (that is red), RGB values (that is rgb(255,0,0)), Hex values, RGBA values, etc.
<rect></rect>
The above fill=”purple” is an example of a presentational attribute. Interestingly, and unlike inline styles like style=”fill: purple” which also happens to be an attribute, presentational attributes can be overriden by CSS styles defined in a stylesheet. So, if you did something like svg { fill: blue; } it would override the purple fill we’ve defined.
Can you give an example of an @media property other than screen?
Yes, there are four types of @media properties (including screen):
all - for all media type devices
print - for printers
speech - for screenreaders that “reads” the page out loud
screen - for computer screens, tablets, smart-phones etc.
Here is an example of print media type’s usage:
@media print {
body {
color: black;
}
}
What are some of the “gotchas” for writing efficient CSS?
Firstly, understand that browsers match selectors from rightmost (key selector) to left. Browsers filter out elements in the DOM according to the key selector and traverse up its parent elements to determine matches. The shorter the length of the selector chain, the faster the browser can determine if that element matches the selector. Hence avoid key selectors that are tag and universal selectors. They match a large number of elements and browsers will have to do more work in determining if the parents do match.
BEM (Block Element Modifier) methodology recommends that everything has a single class, and, where you need hierarchy, that gets baked into the name of the class as well, this naturally makes the selector efficient and easy to override.
Be aware of which CSS properties trigger reflow, repaint, and compositing. Avoid writing styles that change the layout (trigger reflow) where possible.
What are the advantages/disadvantages of using CSS preprocessors?
Advantages:
CSS is made more maintainable.
Easy to write nested selectors.
Variables for consistent theming. Can share theme files across different projects.
Mixins to generate repeated CSS.
Sass features like loops, lists, and maps can make configuration easier and less verbose.
Splitting your code into multiple files. CSS files can be split up too but doing so will require an HTTP request to download each CSS file.
Disadvantages:
Requires tools for preprocessing. Re-compilation time can be slow.
Not writing currently and potentially usable CSS. For example, by using something like postcss-loader with webpack, you can write potentially future-compatible CSS, allowing you to use things like CSS variables instead of Sass variables. Thus, you’re learning new skills that could pay off if/when they become standardized.
Describe what you like and dislike about the CSS preprocessors you have used.
Likes:
Mostly the advantages mentioned above.
Less is written in JavaScript, which plays well with Node.
Dislikes:
I use Sass via node-sass, which is a binding for LibSass written in C++. I have to frequently recompile it when switching between node versions.
In Less, variable names are prefixed with @, which can be confused with native CSS keywords like @media, @import and @font-face rule.
