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Cascading Style Sheets (CSS)
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This lecture will cover…
Introduction to Cascading Style Sheets (CSS) What is CSS? External stylesheets, embedded and inline styles CSS syntax – rules, properties and values Selectors Inheritance and specificity This week’s lab tutorial This week’s reading
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Reminder – HTML element
Content Start tag End tag <header>A heading<header/>
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Cascading stylesheets
CSS is a declarative language for specifying the presentation of HTML elements by a browser CSS has been developed since 1998 by W3C Current stable (signed off) standard is CSS 2.1 CSS 3 is evolving rapidly: many of its properties are supported in all modern browsers; newer properties are implemented by browsers experimentally Reference - CSS properties and browser support
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Web browsers have their own built-in stylesheet
If an HTML document is not linked to a style-sheet the browsers will present the page according to its own stylesheet
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Progressive enhancement
Older browsers don't support CSS 3 – but we can still use most CSS 3 properties If a browser cannot display a CSS 3 property the fallback will usually be acceptable E.g. CSS 3 rounded corners and drop shadow – older browsers will display right angle corners and no shadow, but users can still access the content and HTML structure and the element looks OK One of the reasons why it's good practice to create well-formed, valid HTML and good content FIRST
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CSS 3 CSS 3 – can easily write styles that in the past could only be done with JavaScript, or digital images, e.g. Rounded corners Drop shadow on borders and text Opacity Decorative borders Importing fonts from the web Animation - transitions and transforms
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Cascading stylesheets
CSS has lead to economies in website development and maintenance - Structure of content (HTML) and presentation (CSS) are separate: presentation is controlled by a single stylesheet and can easily be changed Web page accessibility improved - Different stylesheets can be attached to the same HTML document - e.g. a rich graphic style for GUI browsers; a print stylesheet – optimised for readability with navigation elements removed
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Attaching a stylesheet
Best practice is ALWAYS to write CSS rules in an external stylesheet file This is attached to all the HTML documents to which it applies by referring to it by the <link> tag in the document head - <link href="stylesheet.css" rel="stylesheet" type="text/css"> The 'rel' attribute specifies the relationship between the web document and the file it is linking to – i.e. its stylesheet In the index.html file provided in the lab tutorials the link is commented out Copy the 'Getting started' stylesheet into your website folder. You can find this in the Unit 10 Resources post
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Embedded and inline CSS
Embedded stylesheet - the styles are written inside <style> tags in the <head> of the HTML document inline styles - within the <body> of the HTML document ... you should keep this in check and delete them! Embedded and inline styles will over-ride the stylesheet – which makes pages hard to debug, change and maintain The "cascade" – lower level rules over-ride higher level
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CSS syntax A stylesheet consists of a number of rules
These specify how the browser should present the elements in an HTML document A rule consists of two parts - the selector targets an HTML element one or more declarations, inside curly braces, describe how the element will be displayed A declaration also has two parts - a property, i.e. the name of some aspect of presentation – e.g. 'font-size', 'background-color' a value for the property – e.g. '14px', '#FFFFFF'
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all <h1> elements
A CSS rule Selector – targets all <h1> elements Declarations – how the element should be presented; written inside curly brackets h1 { font-family: Arial, Helvetica, sans-serif; color: #000000; font-size: 30px; } Note the punctuation – very important Value of the property Property – an aspect of the element’s presentation # is the hexadecimal colour code for black
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CSS properties CSS syntax is straightforward
The scope of the language comes from the many properties that can be used to specify the presentation of an element When a number of properties are combined in a rule they can be used to create graphic effects like rollover menu buttons and complex layouts Learning CSS basics means learning properties and how to apply them correctly Using Notepad++ - you'll need a reference list of properties, e.g.
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CSS selectors The selector is used to target the HTML element that the rule will style e.g. h1 { font-size: 30px; } The h1 selector targets all <h1> elements There are several different types of selector You will be learning how to use four selector types to target HTML elements
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CSS selectors A selector is a pattern which matches the style rule to the HTML element The universal selector - * - matches any element E.g. * { padding: 0; margin: 0; } There are many kinds of selector – some are quite complex patterns – but we shall focus on the four most basic ones…
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Hexadecimal colour code, start with #
CSS – type selector The type selector matches any element of that type E.g. body is the selector that matches the <body> element type: used to declare some default values for the whole document – such as the page background colour and font properties - body { background-color: #004080; font-family: Verdana, Geneva, sans-serif; color: #FFFFDC; font-size: 12px; } Hexadecimal colour code, start with #
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CSS – inheritance and specificity
The body element selector is always the first rule in the stylesheet after the reset CSS All HTML elements are 'children' of the <body> element, as they are nested inside it: they will inherit its CSS properties … …until a specific rule is written for an element with properties that over-ride the body properties E.g. the rule body { font-size: 16px; } will determine the font- size of all elements, until another rule is created in the stylesheet e.g. h1 { font-size: 48 px; } or footer { font-size: 10px; }
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Hex code shorthand – when code is in 3 pairs
CSS – type selector h1, h2...h6 are the selectors for the <h1>, <h2>…<h6> elements. Apply properties that specify how the element will be styled e.g. font-size, colour, line-height p is the selector that defines the presentation of all paragraph <p> elements e.g. p { color: #F00; } Hex code shorthand – when code is in 3 pairs #FF0000 = red #F00 = red
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Values of more than one word must go in quotation marks
CSS – type selector The html5 <header> has been used several times, to markup all text content that is a heading – for the page header is the selector that will target any element of that type e.g. header { font-family: Georgia, "Times New Roman", serif; margin-top: 4px; margin-bottom: 4px; } Values of more than one word must go in quotation marks
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Syntax is a full stop followed by the class attribute name
CSS - class selector What if we want to style the <headers> differently? Could give all the article headers a class attribute – <header class="post"> And use a class selector to target all elements with that class attribute – .post { background-color: #069; margin-top: 2px; margin-bottom: 2px; } Syntax is a full stop followed by the class attribute name
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CSS - id selector Another option would be to give the header for the top of the page – which has only one instance – an id attribute <header id="page_title"> And use an id selector to target this unique element – #page_title { background-color: #069; margin-top: 10px; margin-bottom: 6px; } Syntax is # followed by the id attribute name. An id can be used only once in an HTML document
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Class or id attribute/selector?
An id attribute must be unique – only used once in an HTML document e.g. <header id="title"> A class attribute can be used one or more times in an HTML document e.g. <article class="post"> It's OK to use a class attribute / CSS selector for unique instances of elements and forget about id attributes /selectors
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CSS – pseudo-class selector
Some elements – such as <a> anchors (links) have certain behaviours when the user interacts with them Pseudo-class selectors target the element in its different states Selector Matched state or behaviour a:link source anchor of a hyperlink a:visited source anchor of a visited link a:hover anchor when the user moves cursor over it a:active anchor when the user clicks down on it a:focus when anchor gains keyboard focus
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CSS – pseudo-class cascade
Anchor pseudo-class selectors must always be written in the stylesheet in this order LVHA – link, visited, hover, active This is because the lower level, more specific CSS rule properties over-ride the properties in the rules above – the cascade If the pseudo-class rules are written in a different order the styles just aren't applied correctly To test link styles interact with them in the browser. Turn off browser history so that visited links do not persist.
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This week’s lab tutorial
You need a page with a range of HTML elements and content to target with CSS selectors before moving on to … CSS Tutorial– introduction to stylesheets - writing rules with the selectors covered in this lecture
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Reading and links Jon Duckett, HTML & CSS: design and build websites. Read Chapter 10 (Introducing CSS) Web Platform Docs – Using selectors - Online – Selectutorial CSS properties reference –
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