Load and execution sequence of a web page

Question

I have done some web based projects  but I don t think too much about the load and execution sequence of an ordinary web page  But now I need to know detail  It s hard to find answers from Google or SO  so I created this question   A sample page is like this    lt html gt    lt head gt     lt script src  jquery js  type  text javascript  gt  lt  script gt     lt script src  abc js  type  text javascript  gt     lt  script gt     lt link rel  stylesheets  type  text css  href  abc css  gt  lt  link gt     lt style gt h2 font-wight bold   lt  style gt     lt script gt      document  ready function             img   attr  src    kkk png            lt  script gt    lt  head gt    lt body gt       lt img id  img  src  abc jpg  style  width 400px height 300px    gt       lt script src  kkk js  type  text javascript  gt  lt  script gt    lt  body gt   lt  html gt    So here are my questions    How does this page load  What is the sequence of the loading  When is the JS code executed   inline and external  When is the CSS executed  applied   When does   document  ready get executed  Will abc jpg be downloaded  Or does it just download kkk png    I have the following understanding    The browser loads the html  DOM  at first  The browser starts to load the external resources from top to bottom  line by line  If a  lt script gt  is met  the loading will be blocked and wait until the JS file is loaded and executed and then continue  Other resources  CSS images  are loaded in parallel and executed if needed  like CSS     Or is it like this   The browser parses the html  DOM  and gets the external resources in an array or stack-like structure  After the html is loaded  the browser starts to load the external resources in the structure in parallel and execute  until all resources are loaded  Then the DOM will be changed corresponding to the user s behaviors depending on the JS   Can anyone give a detailed explanation about what happens when you ve got the response of a html page  Does this vary in different browsers  Any reference about this question   Thanks   EDIT   I did an experiment in Firefox with Firebug  And it shows as the following image

User · Answer

The chosen answer looks like does not apply to modern browsers, at least on Firefox 52. What I observed is that the requests of loading resources like css, javascript are issued before HTML parser reaches the element, for example

<html>
  <head>
    <!-- prints the date before parsing and blocks HTMP parsering -->
    <script>
      console.log("start: " + (new Date()).toISOString());
      for(var i=0; i<1000000000; i++) {};
    </script>

    <script src="jquery.js" type="text/javascript"></script>
    <script src="abc.js" type="text/javascript"></script>
    <link rel="stylesheets" type="text/css" href="abc.css"></link>
    <style>h2{font-wight:bold;}</style>
    <script>
      $(document).ready(function(){
      $("#img").attr("src", "kkk.png");
     });
   </script>
 </head>
 <body>
   <img id="img" src="abc.jpg" style="width:400px;height:300px;"/>
   <script src="kkk.js" type="text/javascript"></script>
   </body>
</html>

What I found that the start time of requests to load css and javascript resources were not being blocked. Looks like Firefox has a HTML scan, and identify key resources(img resource is not included) before starting to parse the HTML.

User · Answer

Open your page in Firefox and get the HTTPFox addon  It will tell you all that you need   Found this on archivist incuito   http   archivist incutio com viewlist css-discuss 76444     When you first request a page  your   browser sends a GET request to the    server  which returns the HTML to the   browser  The browser then starts    parsing the page  possibly before all   of it has been returned        When it finds a reference to an   external entity such as a CSS file  an   image file  a script file  a Flash   file  or anything else external to    the page  either on the same   server domain or not   it prepares to   make  a further GET request for that   resource       However the HTTP standard specifies   that the browser should not make  more   than two concurrent requests to the   same domain  So it puts each  request   to a particular domain in a queue  and   as each entity is  returned it starts   the next one in the queue for that   domain       The time it takes for an entity to be   returned depends on its size  the    load the server is currently   experiencing  and the activity of   every  single machine between the   machine running the browser and the   server   The list of these machines   can in principle be different for   every  request  to the extent that one   image might travel from the USA to me   in  the UK over the Atlantic  while   another from the same server comes out   via the Pacific  Asia and Europe    which takes longer  So you might get a   sequence like the following  where a   page has  in this order  references    to three script files  and five image   files  all of differing sizes          GET script1 and script2  queue request for script3 and images1-5    script2 arrives  it s smaller than script1   GET script3  queue    images1-5    script1 arrives  GET image1  queue images2-5    image1 arrives  GET image2  queue images3-5    script3 fails to arrive due to a network problem  - GET script3 again     automatic retry     image2 arrives  script3 still not here  GET image3  queue images4-5    image 3 arrives  GET image4  queue image5  script3 still on the way    image4 arrives  GET image5    image5 arrives    script3 arrives          In short  any old order  depending on   what the server is doing  what the    rest of the Internet is doing  and   whether or not anything has errors    and has to be re-fetched  This may   seem like a weird way of doing    things  but it would quite literally   be impossible for the Internet  not    just the WWW  to work with any degree   of reliability if it wasn t done  this   way       Also  the browser s internal queue   might not fetch entities in the order    they appear in the page - it s not   required to by any standard        Oh  and don t forget caching  both in   the browser and in caching  proxies   used by ISPs to ease the load on the   network

User · Answer

If you re asking this because you want to speed up your web site  check out Yahoo s page on Best Practices for Speeding Up Your Web Site  It has a lot of best practices for speeding up your web site

User · Answer

1  HTML is downloaded   2  HTML is parsed progressively   When a request for an asset is reached the browser will attempt to download the asset   A default configuration for most HTTP servers and most browsers is to process only two requests in parallel   IE can be reconfigured to downloaded an unlimited number of assets in parallel   Steve Souders has been able to download over 100 requests in parallel on IE   The exception is that script requests block parallel asset requests in IE   This is why it is highly suggested to put all JavaScript in external JavaScript files and put the request just prior to the closing body tag in the HTML   3  Once the HTML is parsed the DOM is rendered   CSS is rendered in parallel to the rendering of the DOM in nearly all user agents   As a result it is strongly recommended to put all CSS code into external CSS files that are requested as high as possible in the  lt head gt  lt  head gt  section of the document   Otherwise the page is rendered up to the occurance of the CSS request position in the DOM and then rendering starts over from the top   4  Only after the DOM is completely rendered and requests for all assets in the page are either resolved or time out does JavaScript execute from the onload event   IE7  and I am not sure about IE8  does not time out assets quickly if an HTTP response is not received from the asset request   This means an asset requested by JavaScript inline to the page  that is JavaScript written into HTML  tags that is not contained in a function  can prevent the execution of the onload event for hours   This problem can be triggered if such inline code exists in the page and fails to execute due to a namespace collision that causes a code crash   Of the above steps the one that is most CPU intensive is the parsing of the DOM CSS   If you want your page to be processed faster then write efficient CSS by eliminating redundent instructions and consolidating CSS instructions into the fewest possible element referrences   Reducing the number of nodes in your DOM tree will also produce faster rendering   Keep in mind that each asset you request from your HTML or even from your CSS JavaScript assets is requested with a separate HTTP header   This consumes bandwidth and requires processing per request   If you want to make your page load as fast as possible then reduce the number of HTTP requests and reduce the size of your HTML   You are not doing your user experience any favors by averaging page weight at 180k from HTML alone   Many developers subscribe to some fallacy that a user makes up their mind about the quality of content on the page in 6 nanoseconds and then purges the DNS query from his server and burns his computer if displeased  so instead they provide the most beautiful possible page at 250k of HTML   Keep your HTML short and sweet so that a user can load your pages faster   Nothing improves the user experience like a fast and responsive web page

User · Answer

AFAIK  the browser  at least Firefox  requests every resource as soon as it parses it  If it encounters an img tag it will request that image as soon as the img tag has been parsed  And that can be even before it has received the totality of the HTML document    that is it could still be downloading the HTML document when that happens   For Firefox  there are browser queues that apply  depending on how they are set in about config  For example it will not attempt to download more then 8 files at once from the same server    the additional requests will be queued  I think there are per-domain limits  per proxy limits  and other stuff  which are documented on the Mozilla website and can be set in about config  I read somewhere that IE has no such limits   The jQuery ready event is fired as soon as the main HTML document has been downloaded and it s DOM parsed  Then the load event is fired once all linked resources  CSS  images  etc   have been downloaded and parsed as well  It is made clear in the jQuery documentation   If you want to control the order in which all that is loaded  I believe the most reliable way to do it is through JavaScript

User · Answer

According to your sample   lt html gt    lt head gt     lt script src  quot jquery js quot  type  quot text javascript quot  gt  lt  script gt     lt script src  quot abc js quot  type  quot text javascript quot  gt     lt  script gt     lt link rel  quot stylesheets quot  type  quot text css quot  href  quot abc css quot  gt  lt  link gt     lt style gt h2 font-wight bold   lt  style gt     lt script gt      document  ready function            quot  img quot   attr  quot src quot    quot kkk png quot            lt  script gt    lt  head gt    lt body gt       lt img id  quot img quot  src  quot abc jpg quot  style  quot width 400px height 300px  quot   gt       lt script src  quot kkk js quot  type  quot text javascript quot  gt  lt  script gt    lt  body gt   lt  html gt   roughly the execution flow is about as follows   The HTML document gets downloaded The parsing of the HTML document starts HTML Parsing reaches  lt script src  quot jquery js quot      jquery js is downloaded and parsed HTML parsing reaches  lt script src  quot abc js quot      abc js is downloaded  parsed and run HTML parsing reaches  lt link href  quot abc css quot      abc css is downloaded and parsed HTML parsing reaches  lt style gt     lt  style gt  Internal CSS rules are parsed and defined HTML parsing reaches  lt script gt     lt  script gt  Internal Javascript is parsed and run HTML Parsing reaches  lt img src  quot abc jpg quot      abc jpg is downloaded and displayed HTML Parsing reaches  lt script src  quot kkk js quot      kkk js is downloaded  parsed and run Parsing of HTML document ends  Note that the download may be asynchronous and non-blocking due to behaviours of the browser  For example  in Firefox there is this setting which limits the number of simultaneous requests per domain  Also depending on whether the component has already been cached or not  the component may not be requested again in a near-future request  If the component has been cached  the component will be loaded from the cache instead of the actual URL  When the parsing is ended and document is ready and loaded  the events onload is fired  Thus when onload is fired  the    quot  img quot   attr  quot src quot   quot kkk png quot    is run  So   Document is ready  onload is fired  Javascript execution hits    quot  img quot   attr  quot src quot    quot kkk png quot    kkk png is downloaded and loads into  img  The   document  ready   event is actually the event fired when all page components are loaded and ready  Read more about it  http   docs jquery com Tutorials Introducing   document  ready   Edit - This portion elaborates more on the parallel or not part  By default  and from my current understanding  browser usually runs each page on 3 ways  HTML parser  Javascript DOM  and CSS  The HTML parser is responsible for parsing and interpreting the markup language and thus must be able to make calls to the other 2 components  For example when the parser comes across this line   lt a href  quot   quot  onclick  quot alert  test   return false  quot  style  quot font-weight bold quot  gt a hypertext link lt  a gt   The parser will make 3 calls  two to Javascript and one to CSS  Firstly  the parser will create this element and register it in the DOM namespace  together with all the attributes related to this element  Secondly  the parser will call to bind the onclick event to this particular element  Lastly  it will make another call to the CSS thread to apply the CSS style to this particular element  The execution is top down and single threaded  Javascript may look multi-threaded  but the fact is that Javascript is single threaded  This is why when loading external javascript file  the parsing of the main HTML page is suspended  However  the CSS files can be download simultaneously because CSS rules are always being applied - meaning to say elements are always repainted with the freshest CSS rules defined - thus making it unblocking  An element will only be available in the DOM after it has been parsed  Thus when working with a specific element  the script is always placed after  or within the window onload event  Script like this will cause error  on jQuery    lt script type  quot text javascript quot  gt     lt   CDATA       alert    quot  mydiv quot   html           gt     lt  script gt   lt div id  quot mydiv quot  gt Hello World lt  div gt   Because when the script is parsed   mydiv element is still not defined  Instead this would work   lt div id  quot mydiv quot  gt Hello World lt  div gt   lt script type  quot text javascript quot  gt     lt   CDATA       alert    quot  mydiv quot   html           gt     lt  script gt   OR  lt script type  quot text javascript quot  gt     lt   CDATA         window  ready function                        alert    quot  mydiv quot   html                                 gt     lt  script gt   lt div id  quot mydiv quot  gt Hello World lt  div gt

User · Answer

Dynatrace AJAX Edition shows you the exact sequence of page loading  parsing and execution

[javascript] Load and execution sequence of a web page?

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