分享给朋友：把视频贴到Blog或BBS&&通用代码: 复 制flash地址: 复 制html代码: 复 制分享视频到站外获取收益&&手机扫码分享视频二维码2小时内有效プリミティブ リンク Primitive Link OP下载至电脑扫码用手机看用或微信扫码在手机上继续观看二维码2小时内有效プリミティブ リンク Primitive Link OP扫码用手机继续看用或微信扫码在手机上继续观看二维码2小时内有效，扫码后可分享给好友没有优酷APP？立即下载请根据您的设备选择下载版本
药品服务许可证(京)-经营-
节目制作经营许可证京字670号
请使用者仔细阅读优酷、、
Copyright(C)2016 优酷
不良信息举报***:新概念第四册课文及翻译(中英)_百度文库
两大类热门资源免费畅读
续费一年阅读会员，立省24元！
新概念第四册课文及翻译(中英)
上传于||暂无简介
阅读已结束，如果下载本文需要使用5下载券
想免费下载本文？
定制HR最喜欢的简历
下载文档到电脑，查找使用更方便
还剩37页未读，继续阅读
定制HR最喜欢的简历
你可能喜欢PRIMITIVE LINK壁纸集1 - 青空动漫
PRIMITIVE LINK壁纸集1
来源：_有爱的 - 青空动漫
点击复制&PRIMITIVE LINK壁纸集1&网址发给好友
浏览：1421 次
相关动漫壁纸
尊重网上道德，遵守中华人民共和国的各项有关法律法规
承担一切因您的行为而直接或间接导致的民事或刑事法律责任
本站管理人员有权保留或删除其管辖留言中的任意内容
本站有权在网站内转载或引用您的评论
参与本评论即表明您已经阅读并接受上述条款1 Introduction
This section is non-normative.
The Resource Priorities specification defines a means for site developers to programmatically give
the User Agent hints on the download priority of a resource. Without knowing the download priority of
a resource, the User Agent will typically download resources in document order. However, the download
order may not be best optimized for improving both the perceived and real page load performance.
By specifying lower priority resources using the
attribute,
the User Agent is able to better optimize download order when the User Agent is in a resource constrained
environment and sooner execute scripts triggered by the load event of the Document.
Executing scripts sooner
Today, most developers trigger script execution based on the load event of the Document. However,
the load event can be delayed by resources that are not necessary for page load visuals and do not interact
with script. For example, a site may have images well below the fold that are delaying
how long it takes the load event of the document to fire, delaying scripts from executing. Specifying
the lazyload attribute on an element downloading a resource allows the User Agent to not block
the load event of the Document on those resources. By specifying the lazyload attribute on
resources that have no interaction with script or are not necessary for above the fold visuals,
scripts can execute sooner, improving real world page load performance.
Optimizing download priority during network contention
Site developers can improve the perception of page load performance by
optimizing the download of resources when there is network resource contention such that downloads for resources
not required to create the above the fold visuals are prioritized lower.
However, determining which resources are required to create the above the fold visuals for a web page
is something the User Agent cannot easily determine on its own. Instead, the site developer is in the
best position to let the User Agent know the relative priorities of resources on their web application.
The following example shows a theoretical web page that doesn't specify the download priority of resources.
In this example, the site developer will use Stylesheets from Styles.css and script within Visuals.js
to re-layout the page such that Logo.png, Header.png, and MainContent.mp4 will be shown above the fold,
whereas AdditionalImages1.png and AdditionalImages2.png will be shown below the fold. Animations.css is used
to display an animation when a user clicks on an image and Analytics.js is used to collect analytics information,
both not executed during the page load.
&!DOCTYPE html&
&link rel="stylesheet" type="text/css" src="Styles.css" /&
&link rel="stylesheet" type="text/css" src="Animations.css" /&
&img id="Logo" src=".../images/Logo.png"/&
&img id="Header" src=".../images/Header.png"/&
&img id="AdditionalImages1" src=".../images/AdditionalImages1.png"/&
&img id="AdditionalImages2" src=".../images/AdditionalImages2.png"/&
&video id="MainContent"&&source src=".../videos/MainContent.mp4"&&/video&
&script src="Visuals.js" &&/script&
&script src="Analytics.js" &&/script&
As the example page has been specified, a User Agent may download resources in the following order: root document,
CSS in document order, JavaScript in document order, and everything else in document order. In this case,
AdditionalImages1.png and AddtionalImages.png will be downloaded before MainContent.mp4, even though they will not
be displayed above the fold when the page loads. Additionally, Animations.css and Analytics.js will be downloaded
prior to the visual aspect of the page, even though they won't be used in the page load sequence.
attribute on a resource will signal to the User Agent that
in cases of network resource contention, it may lower the download priority of that resource.
The following example shows the same theoretical web page but with the lazyload attribute set for
resources whose download priority can be reduced, as they do not impact the visual appearance of the page
above the fold.
&!DOCTYPE html&
&link rel="stylesheet" type="text/css" src="Styles.css" /&
&link rel="stylesheet" type="text/css" src="Animations.css" lazyload /&
&img id="Logo" src=".../images/Logo.png"/&
&img id="Header" src=".../images/Header.png"/&
&img id="AdditionalImages1" src=".../images/AdditionalImages1.png" lazyload /&
&img id="AdditionalImages2" src=".../images/AdditionalImages2.png" lazyload /&
&video id="MainContent"&&source src=".../videos/MainContent.mp4"&&/video&
&script src="Visuals.js" &&/script&
&script src="Analytics.js" lazyload &&/script &
In this example page, using the
User Agent may instead download resources in the following order: root document,
Styles.css, Visuals.js, Logo.png, Header.png, MainContent.mp4, Animations.css, Analytics.js, AdditionalImages1.png
and AdditionalImages2.png.
Proactively loading resources
Some applications require a large number of resources that may not be required immediately. For example,
a Canvas game may have multiple game levels with different assets required for each level.
Instead of slowing down the initial application loading experience by loading all assets during the
page load, developers can use the lazyload attribute on resources that they do not need immediately.
These resources won't block script execution, as the load event of the Document will not be delayed, and in cases
of network resource contention, the User Agent will optimize downloading the higher priority resources.
The following example shows a theoretical game application that is using lazyload to differentiate
between assets that are required more immediately and assets that are not required immediately.
&!DOCTYPE html&
&body onload='init()'&
&canvas id='GameCanvas'&&/canvas&
&img class='Backgrounds' id='BackgroundLevel1' src='Background1.png' /&
&img class='Backgrounds' id='BackgroundLevel2' src='Background2.png' lazyload /&
var assets = [];
var NUM_ASSETS_LEVEL1 = 100;
var NUM_ASSETS_LEVEL2 = 100;
var NUM_ASSETS_GENERAL = 100;
var URL_LEVEL1_ASSETS = "assets\\level1\\";
var URL_LEVEL2_ASSETS = "assets\\level2\\";
var URL_GENERAL_ASSETS = "assets\\general\\";
function init() {
// Load game assets
loadGameAssets();
// Play game
function loadGameAssets() {
// Load assets required for Level 1
for (i = 0; i & NUM_ASSETS_LEVEL1; i++) {
assets[i] = new Image();
assets[i].src = URL_LEVEL1_ASSETS + i + ".png";
// Lazyload assets required for Level 2
for (j = 0; j & NUM_ASSETS_LEVEL2; j++) {
assets[i + j] = new Image();
assets[i + j].setAttribute('lazyload');
assets[i + j].src = URL_LEVEL2_ASSETS + j + ".png";
// Load additional assets required for all levels
for (k = 0; k & NUM_ASSETS_GENERAL; k++) {
assets[i + j + k] = new Image();
assets[i + j + k].src = URL_GENERAL_ASSETS + k + ".png";
In the previous example, downloading the Background2.png specified in markup does not block the load event of the page
as it has been marked with lazyload. Similarly, for the dynamically loaded resources,
if there had been a network resource contention, the User Agent would have prioritized the downloads for level 1
and general assets over level 2 assets.
2 Conformance
requirements
All diagrams, examples, and notes in this specification are non-normative,
as are all sections explicitly marked non-normative. Everything else in this
specification is normative.
The key words "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in the normative parts of this document
are to be interpreted as described in .
For readability, these words do not appear in all uppercase letters in this specification.
Requirements phrased in the imperative as part of algorithms (such as
"strip any leading space characters" or "return false and abort these steps")
are to be interpreted with the meaning of the key word ("must", "should",
"may", etc) used in introducing the algorithm.
Some conformance requirements are phrased as requirements on attributes,
methods or objects. Such requirements are to be interpreted as requirements
on user agents.
Conformance requirements phrased as algorithms or specific steps may be
implemented in any manner, so long as the end result is equivalent. (In
particular, the algorithms defined in this specification are intended to be
easy to follow, and not intended to be performant.)
The IDL fragments in this specification must be interpreted as
required for conforming IDL fragments, as described in the Web IDL
specification.
3 Terminology
The construction "a Foo object", where Foo is actually an interface, is sometimes used instead of
the more accurate "an object implementing the interface Foo".
Network resource contention is defined as when the User Agent determines there is
a conflict over access to network resources. An example of network resource contention is when a User Agent is unable to start downloading a resource because the User Agent has exhausted all available TCP connections.
4 Resource Priorities
4.1 Introduction
This section is non-normative.
This specification defines a means for site developers to programmatically give the User Agent
hints on the download priority of a resource. This will allow User Agents to more efficiently
manage the order in which resources are downloaded when there is
or when the resource is not visible to the user, and allow the load event of the Document
to fire sooner as it will not be blocked for lower priority resources.
4.2 The lazyload attribute
The lazyload attribute is a boolean and IDL attribute
that indicates the priority order in which the User Agent should download the resource associated with
the element in cases of .
If the User Agent determines that there is ,
the User Agent SHOULD delay downloading resources associated with elements that have the
specified on the
element until all other resources associated with elements that do not have the
specified on the
element have started downloading.
An example of network resource contention can be if there is a connection contention where the User
Agent is using all available connections. A User Agent may give available TCP connections,
or give a more optimized TCP connection if there is a choice between available connections,
to resources that don’t have the lazyload attribute.
The lazyload IDL attribute MUST
boolean attribute value.
boolean and IDL attributes are supported
on the following
capable of
resources:
4.3 Elements capable of downloading resources
4.3.1 The img element
Content attributes:
DOM interface:
partial interface HTMLImageElement {
element supports the
attribute.
4.3.2 The audio element
Content attributes:
DOM interface:
partial interface HTMLAudioElement {
element supports the
attribute.
4.3.3 The video element
Content attributes:
DOM interface:
partial interface HTMLVideoElement {
element supports the
attribute.
4.3.4 The script element
Content attributes:
DOM interface:
partial interface HTMLScriptElement {
element supports the
boolean attribute, along with the
boolean attributes, to indicate the order in which the user agent will download and execute the script.
There are a few possible modes that can be selected using these attributes:
attributes
are present, if there is network resource contention the User Agent SHOULD delay downloading the script until after all other
without the
attribute that will be
a resource
have started downloading, and then the script MUST be executed asynchronously, as soon as it is available.
attributes
are present, the
attribute behavior takes precedence and the
attribute will have no effect.
If only the
attribute is present,
the User Agent MUST run the same steps as if both the
attributes
were present.
IDL attribute of the script element is set to false, the
attribute will have no effect.
4.3.5 The link element
Content attributes:
DOM interface:
partial interface HTMLLinkElement {
element supports the
attributes.
The User Agent MUST NOT block rendering on stylesheets that have the
boolean attribute specified on the associated link element or
IDL attribute set to true on the associated link element.
The User Agent parser MUST NOT block launching new script contexts on stylesheets that have the
boolean attribute specified on the associated link element or
IDL attribute set to true on the associated link element.
4.3.6 The embed element
Content attributes:
DOM interface:
partial interface HTMLEmbedElement {
element supports the
attribute.
4.3.7 The iframe element
Content attributes:
DOM interface:
partial interface HTMLIFrameElement {
element supports the
attribute.
4.3.8 The object element
Content attributes:
DOM interface:
partial interface HTMLObjectElement {
element supports the
attribute.
4.3.9 The svg feImage filter primitive
Content attributes:
DOM interface:
partial interface SVGFEImageElement {
primitive supports the
attribute.
attribute is set to true, the
attribute will have no effect.
4.3.10 The svg image element
Content attributes:
DOM interface:
partial interface SVGImageElement
element supports the
attribute.
4.3.11 The svg use element
Content attributes:
DOM interface:
partial interface SVGUseElement {
element supports the
attribute.
4.3.12 The svg script element
Content attributes:
DOM interface:
partial interface SVGScriptElement {
element supports the
attribute.
4.3.13 The svg tref element
Content attributes:
DOM interface:
partial interface SVGTRefElement {
element supports the
attribute.
4.4 Events
4.4.1 The load event
The load event of the Document MUST NOT be blocked by any
content attribute specified or the
IDL attribute set to true.
The load event of an
attribute MUST continue to be blocked until the resource is loaded.
attribute MUST not change when the DOMContentLoaded event is fired or the behavior of the
loading, interactive, and complete states of the Document's
IDL attribute.
4.4.2 The lazyloaded event
Immediately after all
content attribute
specified or
IDL attribute set to true have their resources downloaded, the User Agent MUST queue a task to fire a simple
at the Document. If there are no
content attribute specified or with the
IDL attribute set to true,
the User Agent MUST queue a task to fire the
event at the Document immediately after the load event of the Document has been fired.
5 References
[IETF RFC 2119]
Scott Bradner, Author. Internet Engineering Task Force, March 1997. Available at
, Robin Berjon, et al, Editors. World Wide Web Consortium, August 2013. This version of the HTML5 is available from . The
of HTML is available at http://www.w3.org/html/wg/drafts/html/master/.
, Cameron McCormack, Editor. World Wide Web Consortium, April 2012. This version of the Web IDL specification is available from http://www.w3.org/TR/2012/CR-WebIDL-/. The
is available at http://www.w3.org/TR/WebIDL/.
, Bert Box, et al, Editors. World Wide Web Consortium, June 2011. This version of the CSS 2.1 specification is available from http://www.w3.org/TR/2011/REC-CSS2-/. The
is available at http://www.w3.org/TR/CSS2/.
, Erik Dahlstr?m, et al, Editors. World Wide Web Consortium, August 2011. This version of the SVG 1.1 specification is available from http://www.w3.org/TR/2011/REC-SVG11-/. The
is available at http://www.w3.org/TR/SVG/.
Acknowledgements
We would like to sincerely thank Bruno Racineux, Jake Archibald, Jason Weber, Josh Tumath, Tony Ross, Travis Leithead,
Yoav Weiss, Marcos Caceres, Brian Birtles, Ivan Kotenkov, Erik Dahlstrom, Boris Zbarsky, William Chan, and Shwetank Dixit
to acknowledge their contributions to this work.