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[CSS]

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[Joseph Ong - Harvard University]

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[This is CS50. - CS50.TV]

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>> Today we'll be talking about CSS or Cascading Style Sheets.

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>> So what exactly is CSS?

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Well, let's break the term CSS down into 2 parts:

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Cascading and Style Sheets.

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Cascading is a little bit more complex, and it's something we'll cover in another video.

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But for starters, Style Sheets very much hints at what CSS does.

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It adds styles to the HTML of a web page,

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changing how the web page aesthetically looks.

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This means everything from the font of the text to the positioning of the content on the page

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to other cool things like making the corners of a box rounded or adding shadows to text.

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You can even do crazy things like make things animate on the screen.

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>> So how do we use CSS with HTML?

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Take this poor-looking site I just wrote.

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If Rob were to look at this site right now, he'd probably say something like,

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"Wow! My introduction looks terrible. Joseph, you're an awful designer."

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"Using the default Times font? Helvetica is so much better."

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>> So what might be the simplest way to apply the styling Rob wants?

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The most obvious way lots of people initially wrote CSS

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was to put what we call style declarations right in the element itself

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using the HTML style attribute.

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A style declaration simply consists of the HTML element's CSS property we want to change

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followed by a colon followed by the new value of the property and a semicolon at the end.

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For example, let's say Rob wants a black border around his introduction.

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We first put the style attribute on Rob's div here

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then inside double quotes put a CSS declaration:

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"border: 1px solid black;"

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The 1 pixel refers to the width of the border, the solid refers to the style of the border,

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and the color at the end determines what the border's color is.

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>> If we want multiple styles on an element, write these declarations in sequence.

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For example, if Rob wants his header text in Helvetica with a blue background

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and more space around the text, we can do this:

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style="font-family: Helvetica; background-color: blue; padding: 5px;"

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The last semicolon is actually optional, but people usually keep it in for consistency's sake.

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>> We'll save explaining some of the cooler and more complex CSS properties

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for another video.

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If you have something in mind, just Googling the effect you want followed by CSS

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will probably give you pretty good results.

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The cool thing is that CSS is pretty broad,

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so odds are if there is anything you want to do--within reason--

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CSS can probably do it.

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We call this kind of styling inline styling.

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The element style is, well, in line with the start tag.

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>> For folks who really like to be organized,

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you might start getting a little peeved at how messy this all looks.

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Imagine if you had to do this for every element on the page,

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plus now your HTML and CSS are all intermixed and cluttered.

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Gross, right?

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To fix this, people eventually started catching on to separating their HTML markup

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from their CSS styling by using something called CSS selectors.

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CSS selectors are used to choose elements to which declarations are applied.

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A selector combined with a list of CSS declarations is often referred to as a CSS rule.

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These rules would be put between open and close HTML style tags,

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often in the head of the document.

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>> It's much easier to see with an example,

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so let's start by creating a simple CSS rule.

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First, let's put style tags in the head section of our HTML.

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Next, the selector.

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We'll start off by using one of the simplest selectors, the hash symbol,

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which selects an HTML element by its ID attribute.

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In this case we'll select the div that represents Rob's introduction

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by typing the hash symbol followed by the div's ID, rob.

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Now the declarations.

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We add open and close braces and shift our earlier inline declarations on Rob's div

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inside of these braces, refresh,

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and, cool, Rob's intro still has a black border around it minus the messy inline ugliness.

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>> Now, what if we wanted to select the h1 inside of Rob's intro?

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You might think, "Let's put an ID on it and then move our earlier styles."

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That works, but CSS has other ways of letting you select elements

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by using what we call combinators to mix simple selectors.

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For example, a whitespace character can be used as a combinator

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to specify all instances of 1 selector that live inside of another selector.

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That sounds a lot more complicated than it actually is.

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Here's an example.

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We will type #rob, add a space, and follow it with an h1,

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another simple selector called a tag selector that selects types of elements

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like divs or paragraphs.

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The space combines our 2 simple selectors, applying all CSS declarations

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and the curly braces to h1 tags that live inside of the element with id="rob".

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Just to convince you that it works, I'll change the font color to white, refresh,

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and, look, Rob's header is now white.

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>> All of this work goes to show that by using rules instead of inline styles

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we can get much cleaner code.

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In fact, if this style block starts getting a little bit large,

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I can even pull these styles out into a different file altogether.

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>> To include this new file as CSS in this document I'll just use HTML's link tag.

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Here I'm telling it that I'm linking in an external style sheet, the rel attribute,

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and specifying the path to the file with my href attribute.

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Now my HTML markup and CSS are organized nicely in completely separate files,

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which is almost always the way designers prefer working with HTML and CSS.

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>> In case you're wondering, the simple selectors include ID selectors

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and tag selectors like the ones we just saw

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as well as class selectors for selecting elements with a certain class,

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attribute selectors for selecting elements by other attributes like type="radio"

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for radio button inputs, and pseudoselectors like hover and focus

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for specifying styling when interactions like mousing over an element occur.

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>> The common combinators include whitespace for all children

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and commas to distinguish selectors.

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Others you may encounter include the arrow for direct children only,

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the tilde for all siblings that occur after the element,

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and the plus sign for the 1 sibling that comes immediately after the element.

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>> By combining these selectors and combinators,

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you can broaden the range of styling you can achieve on a given web page

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and write CSS more efficiently.

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With just a couple of lines of CSS you see me typing here,

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I can quickly make something like this look like something like this.

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>> I'm Joseph, and this is CS50.

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>> [CS50.TV]

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>> Uh, where do I start?

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Let me do that without-- [laughs] Okay.

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Add a-- Let me change that wording.

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Ooh. Sorry.