The role of DOCTYPE and the difference between standards mode and compatibility mode

1. ## declaration must be at the head of the HTML document Before the tag,

   tells the browser's parser what document standard to use to parse this document. A non-existent or incorrectly formatted DOCTYPE will cause the document to be rendered 's typesetting and JS operation mode are all run at the highest standards supported by the browser. In Compatibility Mode, pages are displayed in a loosely backwards-compatible manner, simulating the behavior of older browsers to prevent the site from not working. Specific differences:

   1. Box model

   In strict mode: width is the content width, and the real width of the element = width;
   In compatibility mode: width is =width+ padding+border
   2. In compatibility mode, you can set the percentage height and the height and width of inline elements
   In Standards mode, setting wdith and height for inline elements such as span will not take effect, but in compatibility mode, will take effect.
   In standards mode, the height of an element is determined by the content it contains. If the parent element does not set a height, setting a percentage height of the child element is invalid.
   3. Using margin:0 auto to set horizontal centering will not work under IE
   Using margin:0 auto can center the element horizontally in standards mode, but it will not work in compatibility mode (use the text-align attribute) Solution)
   body{text-align:center};#content{text-align:left}
   4. In compatibility mode, the font attributes in Table cannot inherit the settings of the upper layer, and white-space:pre will be invalid. Setting the padding of the image will be invalid
   
   

   The statement must be at the head of the HTML document, before the tag,
   1. Inform the browser's parser to use What document standard parses this document. A non-existent or incorrectly formatted DOCTYPE will cause the document to be rendered

      in compatibility mode. HTML5 is case-insensitive

   declaration is not an HTML tag, but a directive used to tell the browser the current HTMl version
   The HTML layout engine of modern browsers determines whether to use compatibility mode or standards mode to render the document by checking the doctype. Some browsers have a near-standards model.
   The declaration in HTML4.01 points to a DTD. Since HTML4.01 is based on SGML, the DTD specifies markup rules to ensure that the browser renders the content correctly
   HTML5 is not based on SGML, so there is no need to specify a DTD

   ##The difference between standard mode and compatibility mode

   : Standard mode's typesetting and JS operation mode are all run at the highest standards supported by the browser. In Compatibility Mode, pages are displayed in a loosely backwards-compatible manner, simulating the behavior of older browsers to prevent the site from not working. Specific differences: 1. Box model

   In strict mode: width is the content width, and the real width of the element = width;

   In compatibility mode: width is =width+ padding+border
   2. In compatibility mode, you can set the percentage height and the height and width of inline elements
   In Standards mode, setting wdith and height for inline elements such as span will not take effect, but in compatibility mode, will take effect.
   In standards mode, the height of an element is determined by the content it contains. If the parent element does not set a height, setting a percentage height of the child element is invalid.
   3. Using margin:0 auto to set horizontal centering will not work under IE
   Using margin:0 auto can center the element horizontally in standards mode, but it will not work in compatibility mode (use the text-align attribute) Solution)
   body{text-align:center};#content{text-align:left}
   4. In compatibility mode, the font attributes in Table cannot inherit the settings of the upper layer, and white-space:pre will be invalid. Setting the padding of the image will be invalid
   

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