Summary of the core basics of css

Today’s article integrates the previous CSS supplementary knowledge.

I think the previous knowledge about css is a bit confusing to add, so I will sort it out today.

Cascading Style Sheet

What does cascading mean? Why is this word so important to be in its name.

Cascading can simply be understood as conflict resolution.

What is conflict?

means adding the same style to the same element after selecting it using different selectors.

Priority rules can be expressed as

Inline style>ID style>Category style>Tag style

In a complex page, a certain element may Get the style from many places. For example, a certain

-level title of a website is set to use green as a whole, but a special column needs to use blue, so you need to use

in the column. Override common style settings. In a very simple page, it will not be difficult to implement such special requirements,

But if the structure of the website is very complex, it is entirely possible that the code will become very confusing and may not be found

The style of a certain element comes from which rule. Therefore, it is necessary to fully understand the principle of "cascading" in CSS.

Calculating the priority of conflicting styles is a relatively complex process, and it is not just what the above simple priority rule

can fully describe. But readers can follow a general principle, which is "the more special the style, the higher the priority."

As for how this is special and how to position it the more special it is, please read the following content.

Specificity:

Each selector has specificity, and if an element has two or more conflicting attribute declarations, the one with higher specificity wins.

The specificity of a selector is determined by the components of the selector itself. The specificity value is expressed in 4 parts, such as: 0,0,0,0.

The specific specificity of a selector is determined as follows:

For each ID given in the selector Attribute value, add one to the second part: 0,1,0,0.

For each class attribute value, attribute selection or pseudo-class given in the selector, add one to the third part: 0,0,1,0.

For each element and pseudo-element given in the selector, add one to the fourth part: 0,0,0,1.

Combinators and wildcard selectors do not contribute anything to specificity.

But the specificity of the wildcard selector is zero: that is, 0,0,0,0.

The combining symbol doesn't even have zero.

Example:

h1{color:red;} is 0,0,0,1

p em{color:purple} is 0,0,0,2

.grape{color:purple} is 0,0,1,0

p.b e.a{color:red; } 0,0,2,2

#aa {color:red;} 0,1,0,0

p#aa *[href]{color:red;} 0,1,1,1

And the first 0 It is prepared for inline styles, because the earlier it is, the higher the priority is. If the positions of the non-zero numbers are the same, then compare the size of the previous numbers first,

, and then compare downwards until the numbers are not relative. The one with the larger number has higher priority.

That’s why there is the general priority rule above:

Inline style>ID style>Category style>Tag style

They can just be used as each There are also four representatives of some departments, and they are seated according to their numbers.

Important statement !important means to mark the statement you need. It has the highest priority, but it should be placed at the end of the declared value.

Inheritance:

The speciality of inheritance is not even zero, that is, there is no speciality;

There is a big difference between this zero speciality and no speciality. That is, a selector with 0 specificity can add styles to descendants.

Although inheritance can also add styles to descendants, there are restrictions. Only those with the ability to inherit can be added to descendant elements, such as color, etc. Margin, padding and border properties will not be added to descendants.

Those with the same weight will be compared in order. The lower the order, the higher the priority.

Higher specificity is stronger than lower specificity

So pseudo-class declaration order: link-visited-hover-active

LVHA (abbreviation)

When :visited comes after them, since the weights are the same, their order will be looked at, and :visited will win after them.

This problem will not occur if unified attributes are not written. So be careful when writing the same attribute! ! ! !

The above is the core foundation of css.

Expand this knowledge:

Containing block:

The width of the containing block is the width of the parent of the contained element width, and the width of the parent is the

margin-left + border-left + padding-left + width + padding-right + border-right + margin-right=containing block horizontal Width

width, margin-left, and margin-right can all be set to auto;

If margin-left and margin-right are set to auto at the same time, they will be centered, but this is a problem Conditional:

1. This element must be a block-level element and not separated from the document flow

2. This element has set width

margin-top and margin-bottom settings Auto will not center vertically. If set to auto, they will only be equal to zero.

The seven attributes of the total horizontal length. These seven attributes are the attributes of the calculation formula above. Their sum cannot be greater than the total horizontal length, and margin can be a negative value, which does not violate this rule in calculation.

Note: Only margin can be negative.

As for what it looks like when the margin is negative, experience it yourself. This attribute seems quite illegal.

The above summary of the core basics of CSS (recommended) is all the content shared by the editor. I hope it can give you a reference, and I also hope that everyone will support the PHP Chinese website.

For more articles related to the summary of the core basics of css, please pay attention to the PHP Chinese website!