CSS is a powerful tool for developers. It's important that you understand it well before trying to use any library or framework, like Bootstrap or Foundation. Here are some tips to help you get started learning CSS:

Understand the CSS box model.

The CSS box model is a way of understanding how elements are rendered on the page. It's made up of four parts: content, padding, border, and margin. Content is the actual text or image that you see in an element. Padding is extra space around your content—this keeps it from touching other elements that might be next to it. A border is a line surrounding an element (and can also be used to style links). Margins are all about spacing between elements; they're a little trickier because they can't be set directly with CSS but can be influenced by padding or borders within an element.

Use clearfix correctly.

In the CSS world, there are a lot of “clearing” elements that can be used to ensure that text and other elements don't run under floated elements. One such element is called a “clearfix,” which has been around for some time now.

The clearfix hack allows you to make sure that content doesn't run under any floated elements by using a simple combination of CSS rules:

Add a div element with overflow set to hidden after the last child element in your HTML (in this case, it will be your floated element). This makes sure all child elements are contained within this parent div.

Inside this newly added div, add two more declarations—clear: both and height: 0. These will force the browser not to render anything outside the boundaries of this new parent div and also make sure that no space exists between any floated element and its container because it's being cleared out by those two declarations respectively

Know when to use relative, absolute, and fixed position.

Use relative position to make elements move with their parents and siblings. If a parent element has position: relative, all of its children will have position: relative by default.

Use absolute position to make an element appear in a specific place on the page, regardless of where it appears in the document flow. A div with an id or class set to “container” will be positioned absolutely by default, so you can put images or other content inside that div without affecting the layout of your page.

Use fixed position when you want an element to remain in place while scrolling occurs on the page; for example, if you want to create a fixed toolbar at the top or bottom of your site that remains visible as users scroll down/up through content within another section (without having to scroll back up again just because they came too far down).

Know your selectors.

Selectors are used to tell your CSS what to do. They're the way you make your content look good.

The simplest selector is an ID, which uniquely identifies an element on the page. For example, if a page had a paragraph with the ID “my-paragraph”, then that paragraph would only be styled by rules with p#my-paragraph as their selector. IDs are great for when you want to style just one specific thing on a page—like an image or headline—but they can't be reused anywhere else in your CSS file because they're unique to that element only!

Classes are similar but more flexible: they can be reused throughout your stylesheet and across multiple pages of content (as long as all those pages use the same class).

Be consistent with your units.

Consistency is important because it makes your code easier to read and maintain. It will help you avoid bugs and surprises. There are two ways to be consistent: with your units, and with the way you name things.

The first method is pretty straightforward—you can either be consistent with whether or not you use an upper-case letter for a unit name, or you can choose one system of units and stick with it (for example, all of your numbers will be in cm).

The second method is more complicated: names should have a consistent capitalization pattern throughout your code base (if there's no established convention for this; like if it's a new project or there isn't much-shared history between developers).

Grasp the cascade.

The cascade refers to the order in which CSS styles are applied to a document. It's a key concept in understanding how CSS works, and one of its main strengths. However, it can also cause unexpected results when not understood properly.

To get up to speed on cascading stylesheets (CSS), let’s look at some examples that demonstrate the differences between the browser’s default style sheet and an external style sheet:

The browser applies all styles from its own built-in style sheet before any other source of information about what should appear on your web page. This means that if you want your web page text to all be black (#000000) while everything else is white (#FFFFFF), then you need only set this rule once in your external style sheet: body {color: #000000;}

If there isn't anything specific defined in an external source for something like font color or background color—or even element positioning—then the browser will use whatever rules are defined by default within the browser itself (which may vary depending on what devices or versions are being used). For example, if one user has an old version of Windows Internet Explorer 6 installed on their computer but another has newer versions installed with better support for CSS3 properties such as gradients and rounded corners...well...we're not going there!

Organize your code efficiently.

Organizing your CSS code is essential. There are a few things that you can do to help yourself:

Use a CSS preprocessor. This will allow you to write more efficient code, which will save time in the long run and make it easier for you to debug your project. We recommend using Sass or Less.

Comment your code whenever possible so that others can understand what each line means. In addition, if anything ever breaks with no explanation of why that happens, then someone who knows nothing about web development can look at all of your comments and figure out what went wrong because all relevant information was written down for them!

Group similar elements together by using classes rather than IDs (so as not to have conflicts with other libraries). The best way we've found for doing this is by creating separate folders for each type of element (such as buttons) within one main folder called "stylesheets". This keeps everything organized neatly without having too much clutter when looking through files later on!

Think mobile first.

Mobile first is a design approach that puts mobile devices first.

The idea behind it is that because most people now use smartphones, tablets and other mobile devices to access the web, you should design your site with those users in mind.

Mobile first forces you to think about what's important and what's not on your page. You have less space than you do on a desktop computer, so it forces you to be more creative with how you present information and interactivity on the page itself.

Learn CSS well before you try any library like Bootstrap or Foundation

Learning CSS well before you try a library like Bootstrap or Foundation is important for three reasons:

You will understand what is happening under the hood, which will allow you to customize the library to fit your needs. For example, if you have special requirements for responsive images on small screens (e.g., not resizing them), then you don't want those included in the image sprites that Bootstrap provides by default. If you know how CSS works and understand how browsers handle media queries, then it's easy enough to make sure that only one version of each image sprite is downloaded regardless of screen size.

Learning CSS helps with troubleshooting problems when using third-party libraries such as these (and I've had plenty). This can be especially helpful when working remotely with others who aren't familiar with your codebase and so can't easily look at what's broken on their own computers (this has happened twice!).

Conclusion

The most important thing to remember is that CSS is a powerful tool and can make or break your design. It’s not something you should use lightly, and it’s not something you should expect to learn overnight if you haven’t been doing web design for some time already. That said, there are many resources available online if you want to learn more about CSS – so go ahead and start reading!