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Histogram influencing minimizes banding in GIFs and PNGs by weighting color palettes towards problem areas. Lower banding at lower bit-depths makes for smaller images. 
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Dithering uses existing colors to strategically change pixels to minimize apparent banding.
Mickael Ruau's insight:
Used judiciously however, dithering can allow you to use lower bit-depths. A 32-color image at 100% dithering is often smaller than a non-dithered 64-color image (see Figure 3).
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Crop images contextually for faster downloads and higher impact. By cropping maximally and resizing you can convey meaning without slowing down your web pages.
Mickael Ruau's insight:
Be careful when resaving JPEG images. Reoptimizing an existing JPEG can compound compression artifacts. It is possible to transform JPEGs losslessly, however. Lossless transformations (like 90-degree rotations and flips) require the dimensions of the JPEG to be a multiple of the block size (16x16, 16x8, or 8x8 pixels for color JPEGs). Lossless crops are also possible by cropping to block boundaries with specialized software, like JPEG Wizard.
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PNGs are designed to be a more efficient, flexible, and patent-free replacement for GIFs. By replacing your GIFs with PNGs you can speed up your web pages and save bandwidth.
Mickael Ruau's insight:
PNGs uses the deflate compression algorithm typically with a 32KB sliding window. Deflate is an improved version of the Lempel-Ziv compression algorithm (LZ77) used in ZIP and GZIP files (3,4). Created by Phil Katz for version 2 of PKZip, deflate combines LZ77 with Huffman encoding and is 10% to 30% more efficient than LZW at lossless compression. Like gzip, some PNG compression tools have an optional "compression ratio" with values ranging from one to nine. Six is the default. Nine is almost always the best setting for maximum compression ratio. Not surprisingly, images saved as PNGs are typically 10% to 30% smaller than GIFs, although in rare cases they can be larger (5,6). In our tests, we found some images can compress 40% to 50% smaller or more (over 85% in one example) depending on the image. Typically images with large flat areas of color compress better than smooth-toned images with many color transition areas.
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This article is a description of the optimization of the html of Webreference.com
Mickael Ruau's insight:
We created a simple optimizer script to clean up extra spaces and comments (non-SSI comments that is). We run this script as the last step in our CRON that build the home page. This script is available as open source.
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Optimizing web graphics speeds page display by minimizing bit-depths, cropping and combining images, and using the right graphics software.
Mickael Ruau's insight:
The key to optimizing web graphics is to minimize their size, number, and quality, and maximize cropping. Combine adjacent images and use client-side imagemaps instead. Substitute CSS-based rollovers for bandwidth-hungry image-based rollovers. Replace graphic text with styled text. Finally, to minimize HTTP requests, reuse images with the same URL. With all graphics, you can't go wrong if you do the following: Eliminate and combine your images to minimize HTTP requests.Convert graphic text into styled text.Maximize cropping and minimize dimensions.Use the right image format (JPEGs for photographs).Use interlacing with care (avoid interlacing on smaller images and most PNGs).When capturing photographs, use a stable platform, (tripod, image stabilization, or a Gyroscopic stabilizer)Minimize noise, smear, and complexity.Use weighted optimization to reduce less important areas and blur backgrounds (for JPEGs).Minimize colors (bit-depth) for palette-based formats.Use a high-quality quantizer on true-color originals and limit dithering.Set your image resolution to 72dpi.Specify the actual height and width of all images.Use grayscale where possible.Use thumbnails linked to larger images.Experiment with different settings and programs.
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Weighted optimization or regional compression applies different degrees of compression to different areas of your image. By varying the quality within images you can improve file size.
Mickael Ruau's insight:
For JPEGs this technique is actually a fudge that discards higher-frequency detail in less important areas. The resulting JFIF is still a valid file. JPEG2000 includes regional compression as part of the specification. For GIFs or PNGs you can vary the degree of "lossiness" and dithering for different areas. More lossiness in GIFs adds more similar pixel patterns along scan lines, that makes the LZW compression algorithm work more efficiently.
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Create faster loading, better looking graphics for the Web. A concise, step-by-step tutorial on optimizing Web graphics. Includes the 216-color non-dithering palette.
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Le contenu du livre est offert en ligne.
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If you're like most developers, you rely heavily on JavaScript to build interactive and quick-responding web applications. The problem is that all of those lines of JavaScript code can slow down your apps.
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Performance is critical to the success of any web site. In this book, Steve Souders, web performance evangelist at Google and former Chief Performance Yahoo!, provides valuable techniques to help you optimize your site's performance.
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mod_pagespeedis an open-source Apache module created by Google to help Make the Web Faster by rewriting web pages to reduce latency and bandwidth.
Mickael Ruau's insight:
PageSpeed Filter Examples
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When looking for ways to improve your website, how do you decide which changes to make? And which changes to keep?
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Background blurring reduces non-critical details to make the JPEG algorithm work more efficiently. Converting to Lab Color mode allows you to maximize blur for minimum file size.
Mickael Ruau's insight:
A close cousin to regional compression, blurring reduces higher frequency details for smaller JPEG images. Blurring smooths out abrupt color and tonal transitions, making the JPEG algorithm work more efficiently. The amount of savings depends on the amount of blurring, and can range anywhere from 5% to over 20%. The less detail there is in an image, the more efficient the compression.
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Using width and height attributes on images ensures fast webpage display. Conserve bandwidth by using actual size for thumbnail images.
Mickael Ruau's insight:
Most web pages use images to spice up their look. However, not all web pages use width and height to size their graphics and objects. Specifying the width and height of all embedded objects like images and applets allows your user's browser to render your page without waiting for images to download and try on for size.
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Learn how to reduce the number of HTTP requests required by your web pages by combining adjacent images and optionally imagemapping the links.
Mickael Ruau's insight:
Une technique un peu différente des sprites css.
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This article is a description of the optimization of the html of Webreference.com
Mickael Ruau's insight:
HTML Optimization Tips:TRIM THE FAT - If you can remove a design element from your page and the page still works, do itCOMMENTS - Remove all commentsHEADMINIMIZE the size of your page's HEAD, it must be parsed before the page is renderedDELAY script loads until the page loads (place JavaScript at end of BODY, use onload)META TAGS - keep under 200 char, use no commas (just spaces)CONDITIONAL META TAGS - even better, conditionally include META tags only for spidersCOMMENTS - to raise relevance, remove, they don't work anymoreTABLESSimplify complex tables as they display slowerBreak tables up into separate tables, with a simple fast loading table of the top (see webref.com for an example)Use the fast table rendering algorithm with "fixed" style or attribute, TABLE {table-layout:fixed; width: 100%}Use one TR ALIGN=CENTER|RIGHT not many TD ALIGN=CENTER|RIGHTsUse CELLPADDING or CELLSPACING for spacing around tables rather than TDsUse colored table cells instead of graphicsSTYLE SHEETS - Optimize and minimize style sheets, use one character names, use global styles (P, not <P class="body">)FONTS - Substitute style sheets for font tags or vice versa, whichever is shorterIMAGESMINIMIZE the size and number of images in your page. Each one takes a HTTP request with associated latency (CNET says most HTTP servers send up to 4 images at once)Use RELEVANCE-ENHANCED cropping (see Nielsens' book summary)MAXIMIZE REUSE - Reuse images where possible to take advantage of cachingUse WIDTH and HEIGHT attributes with all images, tables, and appletsBACKGROUNDS - remove, substitute page/element background colors, table cellsCOMBINE - adjacent graphics and use imagemaps if nec, minimize those HTTP requestsALT TAGS - Minimize all and remove ALTs from single pixel GIFs**REMOVE FLUFF - Get rid of unnecessary spaces, returns, tabs, quotes* (<A HREF=url> works).DHTML/JAVASCRIPTMULTIMEDIA/COMPLEX DHTML - limit on high traffic pages, optimize, use flash insteadJAVASCRIPT - Hand optimize code (single character vars/functions), refactorEXTERNALIZE SCRIPTS - link to external scripts at the end of the page for delayed load (and also create valid XHTML)ATTRIBUTES - Use defaults where possible, use the shortest attribute available, reuse attributes where possible (TR, CSS)COLORS and CHARACTER ENTITIESNAMES - use named colors rather than hex, when shorter (BODY BGCOLOR=white TEXT=black ...)Use ISO-8859-1 for character entities (can be shorter, more compatible)   = Use three char styles for colors (#FC0 = #FFCC00)WRITE TIGHT - Use short descriptive headlines and leadsURL OPTIMIZATIONUse SHORT URLs, keep file names as short as possibleREDIRECTS - Pull a yahoo, use mod_rewrite and .htaccess or a RewriteMap file or server redirects to create short URLs on your home page (i.e., a href=r/d = dhtml/).QUALIFY domains and directories (i.e., webref.com/html/ not webref.com/html) to minimize latencyIP ADDRESSES - Use IP address for domain names when shorter than the name itself***CGIs - Put as much default parameters and error checking possible within your CGI's, not in your HTMLNO "WWW" - Create a identical A record for the zone, to redirect domain.com to www.domain.com****CLOSING TAGS - Get rid of unnecessary closing tags </LI>, even </BODY>* etc.SET PAGE SIZE LIMITS - of 20-30 K, especially for your home pageTRIAGE - Optimize your busiest pages firstTEST - have real users try your pages with modems, ask them to find something
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Image stabilization creates sharper pictures by damping vibration to avoid motion blur. Gyroscopic stabilizers steady cameras for sharper photographs and videos.
Mickael Ruau's insight:
Sharper stills and steady videos mean higher quality originals and higher compression ratios. The fewer details per frame and fewer changes between frames, the smaller you can make the file. The sharper your original, the smaller it will be when you optimize it for the Web.
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Minimize the number of colors in GIFs and PNGs to optimize file size. Lower bit-depths mean smaller palettes and code widths, which makes for smaller images.
Mickael Ruau's insight:
To minimize the file size of palette-based image formats like GIF and PNG, minimize their bit-depth, or number of colors. Each time you pass a power of two, file size jumps higher. By minimizing the number of colors, and maximizing the flat-color areas you will create smaller, more efficient images destined for the Web.
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GIF Animation. Create the smallest possible animated GIF89as. GIF Animation tutorial.
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Performance Checklist
Peer-to-peer architectures pose their own unique set of performance challenges for the application. Direct, one-to-one communication is relatively straightforward, but things get much more complex when more than two parties are involved, at least as far as performance is concerned.
Mickael Ruau's insight:
A short list of criteria to put on the agenda: Signaling service Use a low-latency transport.Provision sufficient capacity.Consider using signaling over DataChannel once connection is established. Firewall and NAT traversal Provide a STUN server when initiating RTCPeerConnection.Use trickle ICE whenever possible—more signaling, but faster setup.Provide a TURN server for relaying failed peer-to-peer connections.Anticipate and provision sufficient capacity for TURN relays. Data distribution Consider using a supernode or a dedicated intermediary for large multiparty communication.Consider optimizing the received data on the intermediary prior to forwarding it to the other peers. Data efficiency Specify appropriate media constraints for voice and video streams.Optimize binary payloads sent over DataChannel.Consider compressing UTF-8 content sent over DataChannel.Monitor the amount of buffered data on the DataChannel and adapt to changes in the conditions of the network link. Delivery and reliability Use out-of-order delivery to avoid head-of-line blocking.If in-order delivery is used, minimize the message size to reduce the impact of head-of-line blocking.Send small messages (< 1,150 bytes) to minimize the impact of packet loss on fragmented application messages.Set appropriate retransmission count and timeouts for partially reliable delivery. The "right" settings depend on message size, type of application data, and latency between the peers.
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Ads, buttons, widgets—third-party content is popular on today’s websites. JavaScript provides a great way to mashup content from different sources. Unfortunately, in most cases, each JavaScript snippet is capable of bringing down the entire site.
Mickael Ruau's insight:
High Performance Snippets
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How prepared are you to build fast and efficient web applications?
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Want to speed up your web site? This book presents 14 specific rules that will cut 20% to 25% off response time when users request a page. Author Steve Souders, in his job as Chief Performance Yahoo!
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Mickael Ruau's insight:
Here are a few simple optimizations that are a pain to do manually, but that mod_pagespeed excels at: Making changes to the pages built by the Content Management Systems (CMS) with no need to make changes to the CMS itself,Recompressing an image when its HTML context changes to serve only the bytes required (typically tedious to optimize manually), andExtending the cache lifetime of the logo and images of your website to a year, while still allowing you to update these at any time.
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When reducing colors in an image, by default Photoshop uses the entire image to create the color palette. You can optimize this palette by creating selections over troublesome areas. In many cases, histogram influencing allows lower bit-depths, resulting in smaller file sizes.