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Series - Offsetting Targeted Elements Part 2

You can use some well-documented CSS methods to offset an element so that it appears lower down in the browser window when it is targeted. However, these methods can clash with other css, breaking the offset or affecting the element style. In this article, I'll explain what causes these issues and then look for some more robust css methods.

This article extends on part 1, where I defined what a targeted element is, and outlined a common method used to offset the element. I also addressed browser support issues and a css-only way of applying the offset progressively. I closed Part 1 with the following method:

<a href="#the_content">Skip to the content</a> <div id="the_content" class="scroll_target">...</div>

.scroll_target:nth-of-type(1n+0):before { content: " "; display: block; margin-top: -8em; height: 8em; visibility: hidden; }

This solved the inconsistency in browser support for position:fixed (assumed for the header) and the pseudo-element :before. It uses the CSS3 pseudo-class :nth-of-type(1n+0) to apply the offset style. This pseudo-class comes later in browser support than position:fixed and :before, and when support for the pseudo-class drops, the style block won't be applied.

The issues with this method are:

Using top padding or top border on the target element will break the offset.

A background applied to the target element will bleed at the top edge, in other words the background will wrap around the :before pseudo-element (as you can see in the previous example).

If the target element has a relative position, elements above its top edge will be covered by the offset, preventing links and interactions.

First, lets take a close look the top border & padding issue.

Top Padding & Border Break the Offset

When there is a top border or top padding applied to the target element, the CSS offset method looses its effect. To illustrate this, take a look at the example below. The first and third chapter elements don't have a top border or top padding, so their offset works perfectly. The second chapter element has a top border of 1em, which breaks our offset. You can use the links at the top of the page to see where the browser places the page in the window when an element is targeted:

See the Pen Block-style link target offset with border brake by Niall Campbell (@niall-campbell84) on CodePen.

But why does this happen? Well, it is all down to how the CSS offset method works. It relies on the margin of the child element (:before) combining with the top margin of the parent element (the target element). This behaviour is known as margin collapsing. Lets look at how margin collapsing works:

Margin Collapsing

From MDN:

Top and bottom margins of blocks are sometimes combined (collapsed) into a single margin whose size is the largest of the margins combined into it

Margin collapsing occurs when two margins directly contact each other. For instance, the bottom margin of a block element and the top margin of the next block element will normally collapse. When this happens there are three possibilities:

When there are two positive margins (8em & 10em), the result is a single margin of the largest of the two (10em).

When there are two negative margins(-8em & -10em), the result is a single margin of the smallest of the two (-10em).

When there is a positive and a negative margin (-8em & 10em), the result is the sum of the positive margin and the negative margin (10em + -8em = 2em)

The CSS offset method I'm using can have three margins that collapse: the top margin of the target element, the bottom margin of the previous element, and the :before pseudo-element of the target element. So why did all three of those margins collapse? Again from MDN:

If there is no border, padding, inline content, or clearance to separate the margin-top of a block from the margin-top of its first child block, or no border, padding, inline content, height, min-height, or max-height to separate the margin-bottom of a block from the margin-bottom of its last child, then those margins collapse. The collapsed margin ends up outside the parent.

This means that more than two margins can collapse so long as there is no separation between each margin. When there is a top border or top padding to an element, the :before pseudo-elements top margin won't collapse with its parent element's top margin, breaking the offset.

Alternative: Margin Collapsing without :before

One way to get around this is to have a separate offset element before the target element that handles the negative margin, like so:

<a href="#the_content">Skip to the content</a> <div class="scroll_target_collapse">...</div> <div id="the_content" class="scroll_target">...</div>

.scroll_target_collapse:nth-of-type(1n+0) { margin-top: -8em; } .scroll_target:nth-of-type(1n+0):before { content: " "; display: block; height: 8em; visibility: hidden; }

So when there is a border or padding applied to the top of the target element, the offset won’t be broken:

See the Pen Block-style link target offset with separate collapse element by Niall Campbell (@niall-campbell84) on CodePen.

Before I move on I should mention something else about the nature of collapsing margins.

Lets say there is a scenario where three margins collapse and two were negative. For example, the bottom margin of an element was -4em, the bottom margin of the next (empty) element was -8em, and the top margin of the following element was 10em. What happens? Well all of the negative margins are collapsed together and all of the positive margins are collapsed together, then they are added to give the final margin. This would look something like:

(min(-4em,-8em) + max(10em)) = -8em + 10em = 2em

This means that when you use negative margins to move content up the page, its effects will be limited by any negative margins it collapses with. If the -8em negative margin was to compensate for an offset, the result of its effect would only move the content up 4em, rather than 8em, leaving a 4em gap between the first and the last element. This is an issue I can’t resolve, so you will need to add extra css rules if you have this issue. For example:

/* if the previous element has a margin bottom of -4em like: */ .previous_element { margin-bottom: -4em; } /* we will have to add -4em to our target collapse */ .previous_element + .scroll_target_collapse { margin-top: -12em; }

Use the plus (+) selector (called an adjacent sibling selector) to target the next sibling.

Background bleeds into previous element

When a background color or image is declared on the target element, the background appears to bleed into the previous element. I say appears to bleed because it is the target element itself that is over the previous element. This is how the CSS offset method works. It moves the target element up the page 8em using a collapsing margin, then moves the internal content of the element down the page 8em using the height of the :before element:

Skip to the content Back to content linkSome content for the third box. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nulla ut massa sollicitudin, aliquet dolor eu, elementum arcu. Duis consequat finibus ultricies. In ultrices augue ligula. Phasellus rhoncus ultricies nibh ac congue. Aliquam erat volutpat. Some fixed header. Press the back button in the browser or click this link to hide me.

There are a couple of good methods I know of to get around this issue. The first is from Nicolas Gallagher & involves a transparent top border. The second involves a table caption element. I'll leave the second method for the final article as it requires quite a lengthy explanation. So back to the first method:

Alternative: Transparent top border & background-clip

This method comes from Nicolas Gallagher's site, you can view the original method and a thorough explanation here: http://nicolasgallagher.com/jump-links-and-viewport-positioning/

This solution uses a transparent top border to push the content down instead of a :before pseudo element. There is one caveat. The background doesn't stop at the inner edge of the border (the padding-box), it stops at the outer edge of the border (the border-box). If the top border is transparent, the background will still show through. There is, however, a background-clip property that can change where the background stops:

.scroll_target_collapse:nth-of-type(1n+0) { margin-top: -8em; } .scroll_target:nth-of-type(1n+0) { border-top: 8em solid transparent; -webkit-background-clip: padding-box; -moz-background-clip: padding; background-clip: padding-box; }

Support for background-clip is: IE9, FF3.6, Chrome 4 (partial until 15), Safari 3.1 (partial until 7), Opera 10.1, Android 2.1 which is almost all encapsulated by our nth-of-type(1n+0) pseudo-class (the exception being FF 3.5).

See the Pen Block-style link target offset with background-clip by Niall Campbell (@niall-campbell84) on CodePen.

You might notice some unusual behaviour with the left and right borders at their top ends. These borders are chamfered with the top transparent border, creating an angled spike. This isn’t ideal. To keep the borders, there needs to be a workaround. Something like this:

Make sure the bottom, left & right borders of the container are transparent:

.scroll_target:nth-of-type(1n+0) { position: relative; border-color: transparent !important; border-style: solid; border-top-width: 8em; -webkit-background-clip: padding-box; -moz-background-clip: padding; background-clip: padding-box; }

Then position its :before absolutely. Make sure it has a z-index of -1 and that its positioned the border width beyond the container. Then apply the border.

.scroll_target:nth-of-type(1n+0):before { content: “”; position:absolute; top: -2px; left: -2px; right: -2px; bottom: -2px; border: 2px solid #ccc; z-index: -1; }

The z-index of -1 is relative to the z stacking of the content of our container and not the container itself. It makes sure the pseudo-element doesn’t appear above its content, preventing user interaction with the content. You can also use pointer-events for this purpose if you want to keep the :before element above the container (see https://css-tricks.com/almanac/properties/p/pointer-events/) but its support is more limited.

There is an issue with this. position: relative means that if there is any overlap with previous elements there is a chance that some of the previous elements covered will loose their interactivity. In this case, it might be better to switch to a box-shadow property instead of a border property, like so:

.scroll_target:nth-of-type(1n+0) { border-top: 8em solid transparent; box-shadow: 0 0 0 2px #ccc inset; -webkit-background-clip: padding-box; -moz-background-clip: padding; background-clip: padding-box; }

Wow! So thats a lot of work just for a border, and if you’re familiar with object oriented css I may have just made you cry a little, but it looks ok, and it might be an acceptable style to use as a fallback.

In the next article I'll introduce a different method to offset targeted elements with CSS: offsetting elements with table-caption. The table-caption method will attempt to keep all existing styles in place. Hope you enjoyed reading! until next time.

#css #offset links #viewport positioning #tutorial #fixed header

Series: Offsetting Targeted Elements - Part 1

When an element is targeted on a page, the browser attempts to position the page so that the top edge of the element is flush with the top edge of the viewport. This feature can be problematic, especially when using a fixed header. In this series, I’ll dive in and assess methods for moving the element down from the top of the viewport when it’s targeted.

What Is A Targeted Element?

A targeted element can be any element on a page that has an ID that matches the fragment identifier of the URI. A fragment identifier is the part of a URI which starts with a hashtag. Its sometimes referred to as the hash or location hash. For example '#first_id' is the fragment identifier of 'http://bkf0.tumblr.com/#first_id' and targets the element with the id 'first_id' like <p id="first_id"></p>

You can use the href value of a link to change the current target of the page. Just use a fragment idenifier with the ID of the element you want to target. For example:

<a href="#the_next_box">Target the element with the id the_next_box</a> <p id="the_next_box"> This element will become the page target when you click the link above </p>

The above code would work like the following link and element:

Target the element with the id the_next_box This element will become the page target when you click the link above

This is a great feature, but the top of the targeted element will be covered by any content that is in a fixed position at the top of the page. It can get pretty confusing to the user, so we need to find a method offset the targeted element.

Common Method For Offsetting The Targeted Element

Lets look at a pretty common method for offsetting an element. This method comes from Chris Coyer on css-tricks. I like to call this method a block-style offset as it uses a block to move the contents of an element down the page, and a collapsing top margin to move the element up the page.

Assuming that you need to move the content of the element down the page 8em, you would implement this offset like so:

<a href="#the_content">Skip to the content</a> <div id="the_content" class="scroll_target">...</div>

.scroll_target:before { content: " "; display: block; margin-top: -8em; height: 8em; visibility: hidden; }

The above example would work something like the following link and element. Note when you click the link a dummy fixed header should appear to demonstrate the offset.

Skip to the content Some content for the second box. Lorem ipsum dolor sit amet, consectetur adipiscing elit. Nulla ut massa sollicitudin, aliquet dolor eu, elementum arcu. Duis consequat finibus ultricies. In ultrices augue ligula. Phasellus rhoncus ultricies nibh ac congue. Aliquam erat volutpat. Some fixed header. Press the back button in the browser or click this link to hide me.

This method depends on a lot of things going right, so it has a few issues (as outlined by Nicolas Gallagher in his Jump Links and Viewport Positioning article).

The pseudo-element :before needs to be supported & its implied that position:fixed should be supported too.

A background applied to the target element will bleed at the top edge, in other words the background will wrap around the :before pseudo-element (as you can see in the previous example).

Using top padding or top border on the target element will break the offset.

The target elements position must be static, if its relative, the offset will cover some the elements above it (z-index), preventing links and interactions.

Over the next few articles I’ll take a look at solutions to the background bleed, top-padding/border & z-index issues, but for the rest of this article I’ll focus on support.

Browser Support

Consider your website without a fixed header. It still works right? So you should think of your fixed header as a ‘progressive enhancement’ and not a core requirement. When support drops for the things that make it work it shouldn’t make the website unuseable.

In our case, we’re using the pseudo-element (:before). If support for this element fails before support for the fixed header then the targeted content will be hidden behind the fixed header. This is a real issue and occurs in IE where IE7 supports position:fixed, yet support for :before starts at IE8 (see https://kimblim.dk/css-tests/selectors/).

Its difficult to target support for :before without using javascript & negatively affecting the way the page loads, so lets look at an alternative. The :before pseudo-element was specified in CSS2.1, so we could use a pseudo-class selector specified in CSS3 to apply the styles to both the fixed header and :before. Pseudo-classes are CSS selectors that are used to narrow the selected elements to those that match a specific state. If the pseudo-class isn’t supported, the style block it defines to won’t be applied. There are a few combinations of pseudo-class selectors which we can use that will not narrow the selection of elements. They are:

:target & :not(:target): :target is used to style the the element who’s ID matches the window location hash & :not() is used to exclude elements by a selector. Support starts at IE9.

:nth-of-type(odd) & :nth-of-type(even): odd and even are keywords for :nth-of-type which match the odd & even indexed children of the elements parent, where the children are grouped by type. Support starts at IE9 & FF3.5.

:nth-of-type(1n+0): :nth-of-type(an+b) is used to match every nth child of same type which matches the an+b formula, where a represents the cycle size, b represents an offset, and n is a counter. The result of the formula are the indices of the same types for which the style is applied. Here we are selecting every child (1x1+0=1, 1x2+0=2, 1x3+0=3, etc). Support starts at IE9 & FF3.5

Lets look a little deeper into each one:

Progressive Enhancement using :target & :not(:target)

:target and :not(:target) are CSS3 selectors that seem to be supported at the same point in browser development (IE9), both later than :before(IE8) and position:fixed(IE7), but they serve two different purposes so its possible that one would be implemented without the other, but I would hazard a guess that this is very rare.

Support: IE9, FF3, Safari 3, Chrome 5, and Opera 10.1.

You would use these pseudo-class selectors to make a CSS declaration block conditional on their support like so:

.scroll_target:not(:target):before, .scroll_target:target:before { content: " "; display: block; margin-top: -8em; height: 8em; visibility: hidden; }

In our example we could get away with just applying the style using :target, since the only element that we’re concerned with offsetting is the targeted element, but there is an increased risk that the browser will trigger a reflow, which is costly to performance. (see. http://frontendbabel.info/articles/webpage-rendering-101/ && http://www.stubbornella.org/content/2009/03/27/reflows-repaints-css-performance-making-your-javascript-slow/ ). A reflow is the browser computing the layout of the page. In fact, even though we’ve declared a style for both :not(:target) and :target here, a reflow still occurs. This is because reflows are triggered on document scroll and fragment identifier (hash) change.

Then is it necessary to consider minimising reflow? The short answer is yes because the browser does its best to restrict repaint and reflow, caching changes and triggering reflow only on the elements the browser knows will be affected. So it can’t hurt to reduce the changes right? So we use :not(:target) to apply the offset target style to all our elements that aren’t the current target. In theory, nothing changes when a new :target element is activated because the new style has the same properties as the old style, but we can use a more robust solution to this problem.

Progressive Enhancement using :nth-of-type(odd) & :nth-of-type(even)

The pseudo-selectors :nth-of-type(odd) & :nth-of-type(even) are supported at the same point in browser development and serve the similar purposes. The selectors also don’t change context when the documents target is changed, which means the browser shouldn’t have to do any work to recalculate the document flow.

Support: IE9, FF3.5, Safari 3, Chrome 5, and Opera 10.1.

You would use these pseudo-class selectors to make a CSS declaration block conditional on their support like so:

.scroll_target:nth-of-type(odd):before, .scroll_target:nth-of-type(even):before { content: " "; display: block; margin-top: -8em; height: 8em; visibility: hidden; }

We’re using :nth-of-type instead of :nth-child since iOS8 Safari has a bug with :nth-child (see http://stackoverflow.com/questions/26032513/ios8-safari-after-a-pushstate-the-nth-child-selectors-not-works) that might affect the styling if you use pushState in javascript. Also I should note the way this pseudo-class works. :nth-of-type groups types of children together, then applies the style to the nth index of each type. This means that if half of the elements with scroll_target are DIV’s and the other half are P’s and we applied the pseudo-selector :nth-of-type(4), the 4th DIV and the 4th P would be styled. This doesn’t really affect our code, but could cause a headache or two if you were abstracting it to suit your needs.

Progressive Enhancement using :nth-of-type(1n+0)

This is probably the most compact pseudo-class that doesn’t narrow the selection of elements. However its a bit complex. :nth-of-type(1n+0) also allows for formulas, in the form of (an+b). The result of which is the index of the child we’re targeting. n is our counter (1,2,3,4,…etc), a multiplies the counter and b adds to the counter. For example the formula (1n+2) would select:

1x1+2 = 3 = the third child of type 1x2+2 = 4 = the fourth child of type 1x3+2 = 5 = the fifth child of type 1x4+2 = 6 = the sixth child of type …etc…

This selects every child of type except the first two. We want to select every child so lets set b to 0 (1n+0). We could write it as simply (n) but Android 4.3 doesn’t like that (see https://developer.mozilla.org/en/docs/Web/CSS/:nth-child).

Support: IE9, FF3.5, Safari 3, Chrome 5, and Opera 10.1.

You would use :nth-of-type(1n+0) to make a CSS declaration block conditional on its support like so:

.scroll_target:nth-of-type(1n+0):before { content: " "; display: block; margin-top: -8em; height: 8em; visibility: hidden; }

This is the most compact method I know of applying the offset target and fixed header progressively using only css that minimises the effect of browser reflow when the element is targeted.

In the next article I’ll talk about why adding a top border or top padding to the target element breaks the offset and address some solutions to this. Later I’ll dig into to background bleed & z-index and offer up some different solutions to fix those. But If you’ve got any questions about anything I’ve mentioned, or further insights, just drop me a line using the ask link above.

Thanks for reading -niall campbell

#css #sticky header #fixed header #offset links #tutorial

Series - Offsetting Targeted Elements Part 2

<a href="#the_content">Skip to the content</a> <div id="the_content" class="scroll_target">...</div>

.scroll_target:nth-of-type(1n+0):before { content: " "; display: block; margin-top: -8em; height: 8em; visibility: hidden; }

The issues with this method are:

Using top padding or top border on the target element will break the offset.

A background applied to the target element will bleed at the top edge, in other words the background will wrap around the :before pseudo-element (as you can see in the previous example).

If the target element has a relative position, elements above its top edge will be covered by the offset, preventing links and interactions.

First, lets take a close look the top border & padding issue.

Top Padding & Border Break the Offset

See the Pen Block-style link target offset with border brake by Niall Campbell (@niall-campbell84) on CodePen.

Margin Collapsing

From MDN:

Top and bottom margins of blocks are sometimes combined (collapsed) into a single margin whose size is the largest of the margins combined into it

When there are two positive margins (8em & 10em), the result is a single margin of the largest of the two (10em).

When there are two negative margins(-8em & -10em), the result is a single margin of the smallest of the two (-10em).

When there is a positive and a negative margin (-8em & 10em), the result is the sum of the positive margin and the negative margin (10em + -8em = 2em)

Alternative: Margin Collapsing without :before

One way to get around this is to have a separate offset element before the target element that handles the negative margin, like so:

<a href="#the_content">Skip to the content</a> <div class="scroll_target_collapse">...</div> <div id="the_content" class="scroll_target">...</div>

.scroll_target_collapse:nth-of-type(1n+0) { margin-top: -8em; } .scroll_target:nth-of-type(1n+0):before { content: " "; display: block; height: 8em; visibility: hidden; }

So when there is a border or padding applied to the top of the target element, the offset won’t be broken:

See the Pen Block-style link target offset with separate collapse element by Niall Campbell (@niall-campbell84) on CodePen.

Before I move on I should mention something else about the nature of collapsing margins.

(min(-4em,-8em) + max(10em)) = -8em + 10em = 2em

Use the plus (+) selector (called an adjacent sibling selector) to target the next sibling.

Background bleeds into previous element

Alternative: Transparent top border & background-clip

This method comes from Nicolas Gallagher's site, you can view the original method and a thorough explanation here: http://nicolasgallagher.com/jump-links-and-viewport-positioning/

See the Pen Block-style link target offset with background-clip by Niall Campbell (@niall-campbell84) on CodePen.

Make sure the bottom, left & right borders of the container are transparent:

Then position its :before absolutely. Make sure it has a z-index of -1 and that its positioned the border width beyond the container. Then apply the border.

.scroll_target:nth-of-type(1n+0):before { content: “”; position:absolute; top: -2px; left: -2px; right: -2px; bottom: -2px; border: 2px solid #ccc; z-index: -1; }

#css #offset links #viewport positioning #tutorial #fixed header

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