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Queries","container-queries",{"id":635,"title":636,"slug":637,"order_index":233,"cornerstone":52,"reading_time_minutes":97},140,"Responsive Navigation Pattern","responsive-navigation-pattern",{"id":150,"name":639,"slug":640,"order_index":368,"topics":641},"Transitions & Animation","transitions-animation",[642,646,650,654,658,662,666],{"id":643,"title":644,"slug":645,"order_index":238,"cornerstone":52,"reading_time_minutes":65},141,"CSS Transitions: Smooth State Changes with transition, @starting-style and allow-discrete","css-transitions",{"id":647,"title":648,"slug":649,"order_index":247,"cornerstone":52,"reading_time_minutes":61},142,"CSS 2D Transforms: translate, rotate, scale, skew and transform-origin","css-2d-transforms",{"id":651,"title":652,"slug":653,"order_index":252,"cornerstone":52,"reading_time_minutes":65},143,"CSS 3D Transforms: perspective, preserve-3d, rotateX\u002FY and backface-visibility","css-3d-transforms",{"id":655,"title":656,"slug":657,"order_index":261,"cornerstone":52,"reading_time_minutes":65},144,"CSS Keyframe Animations: @keyframes, animation shorthand, fill-mode and @property","css-keyframe-animations",{"id":659,"title":660,"slug":661,"order_index":266,"cornerstone":52,"reading_time_minutes":65},145,"CSS Scroll-Driven Animations: animation-timeline, scroll() and view()","css-scroll-driven-animations",{"id":663,"title":664,"slug":665,"order_index":271,"cornerstone":52,"reading_time_minutes":65},146,"CSS View Transitions API: Progressive Enhancement for Page and State Animations","css-view-transitions",{"id":667,"title":668,"slug":669,"order_index":276,"cornerstone":52,"reading_time_minutes":69},147,"Motion Accessibility and Performance: prefers-reduced-motion, will-change and Smooth 60fps Animations","motion-accessibility-prefers-reduced-motion-performance",{"data":671},{"id":507,"title":508,"slug":509,"content":672,"course":673,"subcategory":674,"author":675,"reading_time_minutes":65,"cornerstone":52,"schema_type":681,"faq_json":5,"video_url":5,"canonical_url":682,"meta_title":683,"meta_description":684,"og_image":5,"toc_json":685,"code_examples":746,"quiz":5,"exercises":767,"published_at":5,"updated_at":781,"prev":782,"next":783},"\u003Ch2 id=\"css-box-sizing-content-box-vs-border-box-explained\">CSS box-sizing: content-box vs border-box Explained\u003C\u002Fh2>\u003Cp>The \u003Cstrong>CSS box-sizing\u003C\u002Fstrong> property decides what the \u003Ccode>width\u003C\u002Fcode> and \u003Ccode>height\u003C\u002Fcode> of an element actually measure. It is a tiny property with a huge effect: it is the reason a \u003Ccode>width: 100%\u003C\u002Fcode> element sometimes overflows its parent, and the reason almost every modern stylesheet starts with a \u003Ccode>box-sizing: border-box\u003C\u002Fcode> reset.\u003C\u002Fp>\u003Ch3 id=\"what-you-will-learn\">What You Will Learn\u003C\u002Fh3>\u003Cul>\u003Cli>What \u003Ccode>content-box\u003C\u002Fcode> and \u003Ccode>border-box\u003C\u002Fcode> mean, with exact numbers\u003C\u002Fli>\u003Cli>How the content area shrinks in border-box mode\u003C\u002Fli>\u003Cli>The universal reset and the inherit pattern\u003C\u002Fli>\u003Cli>Which form elements already use border-box by default\u003C\u002Fli>\u003Cli>Using box-sizing with percentages, \u003Ccode>calc()\u003C\u002Fcode> and \u003Ccode>min-height\u003C\u002Fcode>\u003C\u002Fli>\u003Cli>How to read sizes in JavaScript: \u003Ccode>offsetWidth\u003C\u002Fcode>, \u003Ccode>clientWidth\u003C\u002Fcode>, \u003Ccode>getBoundingClientRect()\u003C\u002Fcode>\u003C\u002Fli>\u003C\u002Ful>\u003Ch3 id=\"the-two-values\">The Two Values\u003C\u002Fh3>\u003Cul>\u003Cli>\u003Ccode>box-sizing: content-box\u003C\u002Fcode> (initial value): \u003Ccode>width\u003C\u002Fcode> and \u003Ccode>height\u003C\u002Fcode> set the size of the \u003Cstrong>content box only\u003C\u002Fstrong>. Padding and border are added outside it.\u003C\u002Fli>\u003Cli>\u003Ccode>box-sizing: border-box\u003C\u002Fcode>: \u003Ccode>width\u003C\u002Fcode> and \u003Ccode>height\u003C\u002Fcode> include \u003Cstrong>content + padding + border\u003C\u002Fstrong>. Margin is still outside.\u003C\u002Fli>\u003C\u002Ful>\u003Ch3 id=\"numbers-side-by-side\">Numbers Side by Side\u003C\u002Fh3>\u003Cp>Take an element with \u003Ccode>width: 300px; padding: 20px; border: 5px solid;\u003C\u002Fcode>.\u003C\u002Fp>\u003Cpre>content-box: content = 300px, total width = 300 + 40 + 10 = 350px\nborder-box : total width = 300px, content = 300 - 40 - 10 = 250px\u003C\u002Fpre>\u003Cp>In border-box mode the browser keeps the outer size you asked for and shrinks the content area to make room. If padding and border become larger than the declared width, the content width is clamped to 0 and the box grows to fit them.\u003C\u002Fp>\u003Ch3 id=\"why-border-box-makes-layouts-predictable\">Why border-box Makes Layouts Predictable\u003C\u002Fh3>\u003Cul>\u003Cli>\u003Ccode>width: 100%\u003C\u002Fcode> plus padding no longer overflows the parent.\u003C\u002Fli>\u003Cli>Two columns of \u003Ccode>width: 50%\u003C\u002Fcode> with padding and border still fit on one row.\u003C\u002Fli>\u003Cli>Changing padding or border in a component does not change its outer size, so it does not push neighbours around.\u003C\u002Fli>\u003Cli>Design tools such as Figma measure boxes in the border-box way, so your CSS matches the design numbers directly.\u003C\u002Fli>\u003C\u002Ful>\u003Ch3 id=\"the-universal-reset\">The Universal Reset\u003C\u002Fh3>\u003Cpre>*, *::before, *::after {\n  box-sizing: border-box;\n}\u003C\u002Fpre>\u003Cp>Including the pseudo-elements \u003Ccode>::before\u003C\u002Fcode> and \u003Ccode>::after\u003C\u002Fcode> matters, because the plain \u003Ccode>*\u003C\u002Fcode> selector does not match them.\u003C\u002Fp>\u003Ch3 id=\"the-inherit-pattern\">The Inherit Pattern\u003C\u002Fh3>\u003Cpre>html { box-sizing: border-box; }\n*, *::before, *::after { box-sizing: inherit; }\u003C\u002Fpre>\u003Cp>Here every element inherits from the root. A third-party widget that needs content-box can opt out with \u003Ccode>.widget { box-sizing: content-box; }\u003C\u002Fcode> and all its children inherit that choice automatically. This is useful when embedding old plugins.\u003C\u002Fp>\u003Ch3 id=\"elements-that-default-to-border-box\">Elements That Default to border-box\u003C\u002Fh3>\u003Cp>Browsers already style several form controls with border-box, for example \u003Ccode>input[type=button]\u003C\u002Fcode>, \u003Ccode>input[type=reset]\u003C\u002Fcode>, \u003Ccode>input[type=submit]\u003C\u002Fcode>, \u003Ccode>button\u003C\u002Fcode>, \u003Ccode>select\u003C\u002Fcode> and \u003Ccode>meter\u003C\u002Fcode>\u002F\u003Ccode>progress\u003C\u002Fcode> in most engines. Text inputs and textareas usually still default to content-box, which is why a \u003Ccode>width: 100%\u003C\u002Fcode> input often overflows its form until the reset is applied.\u003C\u002Fp>\u003Ch3 id=\"box-sizing-with-percentages-and-calc\">box-sizing with Percentages and calc()\u003C\u002Fh3>\u003Cul>\u003Cli>Percentage widths are always resolved against the containing block width. With border-box, the resulting number already includes padding and border.\u003C\u002Fli>\u003Cli>In content-box mode you can write \u003Ccode>width: calc(100% - 2 * 20px - 2 * 5px)\u003C\u002Fcode> to compensate, but border-box removes the need for this.\u003C\u002Fli>\u003Cli>\u003Ccode>min-width\u003C\u002Fcode>, \u003Ccode>max-width\u003C\u002Fcode>, \u003Ccode>min-height\u003C\u002Fcode> and \u003Ccode>max-height\u003C\u002Fcode> follow the same box-sizing value as \u003Ccode>width\u003C\u002Fcode> and \u003Ccode>height\u003C\u002Fcode>.\u003C\u002Fli>\u003C\u002Ful>\u003Ch3 id=\"what-about-padding-box\">What About padding-box?\u003C\u002Fh3>\u003Cp>An older draft value \u003Ccode>padding-box\u003C\u002Fcode> existed in Firefox but was removed. Today only \u003Ccode>content-box\u003C\u002Fcode> and \u003Ccode>border-box\u003C\u002Fcode> are supported, so do not use \u003Ccode>padding-box\u003C\u002Fcode>.\u003C\u002Fp>\u003Ch3 id=\"reading-sizes-in-javascript\">Reading Sizes in JavaScript\u003C\u002Fh3>\u003Cul>\u003Cli>\u003Ccode>offsetWidth\u003C\u002Fcode> \u002F \u003Ccode>offsetHeight\u003C\u002Fcode>: border box size (content + padding + border, plus scrollbar). Rounded to integers.\u003C\u002Fli>\u003Cli>\u003Ccode>clientWidth\u003C\u002Fcode> \u002F \u003Ccode>clientHeight\u003C\u002Fcode>: padding box size (content + padding, excluding border and scrollbar).\u003C\u002Fli>\u003Cli>\u003Ccode>getBoundingClientRect()\u003C\u002Fcode>: border box size as decimals, and affected by transforms.\u003C\u002Fli>\u003Cli>\u003Ccode>getComputedStyle(el).width\u003C\u002Fcode>: returns the value in the units of the current \u003Ccode>box-sizing\u003C\u002Fcode>. In border-box mode it includes padding and border.\u003C\u002Fli>\u003C\u002Ful>\u003Ch3 id=\"common-mistakes\">Common Mistakes\u003C\u002Fh3>\u003Cul>\u003Cli>Applying the reset only to \u003Ccode>div\u003C\u002Fcode> or \u003Ccode>body\u003C\u002Fcode> and forgetting inputs and pseudo-elements.\u003C\u002Fli>\u003Cli>Mixing border-box and content-box components and then wondering why widths do not add up.\u003C\u002Fli>\u003Cli>Assuming margin is included in border-box. It never is.\u003C\u002Fli>\u003Cli>Setting padding bigger than the declared width in border-box mode and being surprised the box grows.\u003C\u002Fli>\u003C\u002Ful>\u003Ch3 id=\"best-practices\">Best Practices\u003C\u002Fh3>\u003Cul>\u003Cli>Put the border-box reset at the very top of your global stylesheet.\u003C\u002Fli>\u003Cli>Use the inherit pattern when you must embed third-party components.\u003C\u002Fli>\u003Cli>Verify real sizes in DevTools instead of guessing.\u003C\u002Fli>\u003C\u002Ful>\u003Ch3 id=\"frequently-asked-questions\">Frequently Asked Questions\u003C\u002Fh3>\u003Ch4 id=\"what-is-the-default-value-of-box-sizing\">What is the default value of box-sizing?\u003C\u002Fh4>\u003Cp>The default is \u003Ccode>content-box\u003C\u002Fcode> for most elements.\u003C\u002Fp>\u003Ch4 id=\"does-border-box-include-margin\">Does border-box include margin?\u003C\u002Fh4>\u003Cp>No. Margin always stays outside the box, in every mode.\u003C\u002Fp>\u003Ch4 id=\"is-box-sizing-inherited\">Is box-sizing inherited?\u003C\u002Fh4>\u003Cp>No, it is not inherited by default. That is why the reset uses the universal selector, or \u003Ccode>box-sizing: inherit\u003C\u002Fcode> explicitly.\u003C\u002Fp>\u003Ch4 id=\"does-border-box-hurt-performance\">Does border-box hurt performance?\u003C\u002Fh4>\u003Cp>No. The cost difference is negligible, and it is the default choice of Tailwind CSS, Bootstrap and most modern frameworks.\u003C\u002Fp>\u003Ch3 id=\"key-takeaways\">Key Takeaways\u003C\u002Fh3>\u003Cul>\u003Cli>content-box: width = content only. border-box: width = content + padding + border.\u003C\u002Fli>\u003Cli>Use \u003Ccode>*, *::before, *::after { box-sizing: border-box; }\u003C\u002Fcode> in every project.\u003C\u002Fli>\u003Cli>The property is not inherited, so set it on all elements or use the inherit pattern.\u003C\u002Fli>\u003Cli>Margin is never part of the box-sizing calculation.\u003C\u002Fli>\u003C\u002Ful>\u003Cdiv class=\"try-it-yourself language-html\">\u003Cpre>\u003Ccode><style>\n.a, .b { width: 300px; padding: 20px; border: 5px solid #2c3e50; margin-bottom: 16px; background: #ecf0f1; }\n.a { box-sizing: content-box; }\n.b { box-sizing: border-box; }\n<\u002Fstyle>\n<div class='a'>content-box: total width 350px<\u002Fdiv>\n<div class='b'>border-box: total width 300px<\u002Fdiv>\u003C\u002Fcode>\u003C\u002Fpre>\u003C\u002Fdiv>\u003Cdiv class=\"try-it-yourself language-css\">\u003Cpre>\u003Ccode>*, *::before, *::after {\n  box-sizing: border-box;\n}\u003C\u002Fcode>\u003C\u002Fpre>\u003C\u002Fdiv>\u003Cdiv class=\"try-it-yourself language-css\">\u003Cpre>\u003Ccode>html { box-sizing: border-box; }\n*, *::before, *::after { box-sizing: inherit; }\n\n\u002F* A legacy widget can opt out locally *\u002F\n.legacy-widget { box-sizing: content-box; }\u003C\u002Fcode>\u003C\u002Fpre>\u003C\u002Fdiv>\u003Cdiv class=\"try-it-yourself language-html\">\u003Cpre>\u003Ccode><style>\n.parent { width: 400px; background: #eee; padding: 10px; }\ninput { width: 100%; padding: 12px; border: 3px solid #3498db; margin-bottom: 10px; }\n.cb { box-sizing: content-box; }\n.bb { box-sizing: border-box; }\n<\u002Fstyle>\n<div class='parent'>\n  <input class='cb' placeholder='content-box: overflows the parent'>\n  <input class='bb' placeholder='border-box: fits exactly'>\n<\u002Fdiv>\u003C\u002Fcode>\u003C\u002Fpre>\u003C\u002Fdiv>\u003Cdiv class=\"try-it-yourself language-html\">\u003Cpre>\u003Ccode><style>\n.col { float: left; width: 50%; padding: 20px; border: 2px solid #8e44ad; background: #f5eef8; box-sizing: border-box; }\n.clearfix::after { content: ''; display: block; clear: both; }\n<\u002Fstyle>\n<div class='clearfix'>\n  <div class='col'>Column 1 (50%)<\u002Fdiv>\n  <div class='col'>Column 2 (50%)<\u002Fdiv>\n<\u002Fdiv>\u003C\u002Fcode>\u003C\u002Fpre>\u003C\u002Fdiv>\u003Cdiv class=\"try-it-yourself language-javascript\">\u003Cpre>\u003Ccode>const el = document.createElement('div');\nel.style.cssText = 'width:300px;height:100px;padding:20px;border:5px solid black;box-sizing:content-box;';\ndocument.body.appendChild(el);\nconsole.log('offsetWidth  (border box):', el.offsetWidth);\nconsole.log('clientWidth  (padding box):', el.clientWidth);\nconsole.log('rect.width   (border box):', el.getBoundingClientRect().width);\nel.style.boxSizing = 'border-box';\nconsole.log('--- after border-box ---');\nconsole.log('offsetWidth:', el.offsetWidth);\nconsole.log('clientWidth:', el.clientWidth);\nconsole.log('computed width:', getComputedStyle(el).width);\nel.remove();\u003C\u002Fcode>\u003C\u002Fpre>\u003C\u002Fdiv>",{"name":9,"slug":10},{"name":487,"slug":488},{"slug":676,"display_name":677,"job_title":5,"short_bio":5,"expertise_tags":678,"headshot":5,"long_bio":5,"credentials":5,"twitter_url":5,"linkedin_url":5,"github_url":5,"personal_website_url":5,"other_sameas_urls":5,"topics":679,"posts":680},"badar-khalil","Badar Khalil",[],[],[],"Article","https:\u002F\u002Fwpfsharp.com\u002Ftopics\u002Fcss-box-sizing","CSS box-sizing Explained: content-box vs border-box","Understand CSS box-sizing with clear examples: content-box vs border-box, the universal reset, inherit pattern, form controls and JavaScript size APIs.",[686],{"id":687,"text":688,"level":12,"children":689},"css-box-sizing-content-box-vs-border-box-explained","CSS box-sizing: content-box vs border-box Explained",[690,694,698,702,706,710,714,718,722,726,730,734,738,742],{"id":691,"text":692,"level":57,"children":693},"what-you-will-learn","What You Will Learn",[],{"id":695,"text":696,"level":57,"children":697},"the-two-values","The Two Values",[],{"id":699,"text":700,"level":57,"children":701},"numbers-side-by-side","Numbers Side by Side",[],{"id":703,"text":704,"level":57,"children":705},"why-border-box-makes-layouts-predictable","Why border-box Makes Layouts Predictable",[],{"id":707,"text":708,"level":57,"children":709},"the-universal-reset","The Universal Reset",[],{"id":711,"text":712,"level":57,"children":713},"the-inherit-pattern","The Inherit Pattern",[],{"id":715,"text":716,"level":57,"children":717},"elements-that-default-to-border-box","Elements That Default to border-box",[],{"id":719,"text":720,"level":57,"children":721},"box-sizing-with-percentages-and-calc","box-sizing with Percentages and calc()",[],{"id":723,"text":724,"level":57,"children":725},"what-about-padding-box","What About padding-box?",[],{"id":727,"text":728,"level":57,"children":729},"reading-sizes-in-javascript","Reading Sizes in JavaScript",[],{"id":731,"text":732,"level":57,"children":733},"common-mistakes","Common Mistakes",[],{"id":735,"text":736,"level":57,"children":737},"best-practices","Best Practices",[],{"id":739,"text":740,"level":57,"children":741},"frequently-asked-questions","Frequently Asked Questions",[],{"id":743,"text":744,"level":57,"children":745},"key-takeaways","Key Takeaways",[],[747,751,754,757,760,763],{"id":748,"language":16,"code":749,"is_editable":750},327,"\u003Cstyle>\n.a, .b { width: 300px; padding: 20px; border: 5px solid #2c3e50; margin-bottom: 16px; background: #ecf0f1; }\n.a { box-sizing: content-box; }\n.b { box-sizing: border-box; }\n\u003C\u002Fstyle>\n\u003Cdiv class='a'>content-box: total width 350px\u003C\u002Fdiv>\n\u003Cdiv class='b'>border-box: total width 300px\u003C\u002Fdiv>",true,{"id":752,"language":10,"code":753,"is_editable":750},328,"*, *::before, *::after {\n  box-sizing: border-box;\n}",{"id":755,"language":10,"code":756,"is_editable":750},329,"html { box-sizing: border-box; }\n*, *::before, *::after { box-sizing: inherit; }\n\n\u002F* A legacy widget can opt out locally *\u002F\n.legacy-widget { box-sizing: content-box; }",{"id":758,"language":16,"code":759,"is_editable":750},330,"\u003Cstyle>\n.parent { width: 400px; background: #eee; padding: 10px; }\ninput { width: 100%; padding: 12px; border: 3px solid #3498db; margin-bottom: 10px; }\n.cb { box-sizing: content-box; }\n.bb { box-sizing: border-box; }\n\u003C\u002Fstyle>\n\u003Cdiv class='parent'>\n  \u003Cinput class='cb' placeholder='content-box: overflows the parent'>\n  \u003Cinput class='bb' placeholder='border-box: fits exactly'>\n\u003C\u002Fdiv>",{"id":761,"language":16,"code":762,"is_editable":750},331,"\u003Cstyle>\n.col { float: left; width: 50%; padding: 20px; border: 2px solid #8e44ad; background: #f5eef8; box-sizing: border-box; }\n.clearfix::after { content: ''; display: block; clear: both; }\n\u003C\u002Fstyle>\n\u003Cdiv class='clearfix'>\n  \u003Cdiv class='col'>Column 1 (50%)\u003C\u002Fdiv>\n  \u003Cdiv class='col'>Column 2 (50%)\u003C\u002Fdiv>\n\u003C\u002Fdiv>",{"id":764,"language":765,"code":766,"is_editable":750},332,"javascript","const el = document.createElement('div');\nel.style.cssText = 'width:300px;height:100px;padding:20px;border:5px solid black;box-sizing:content-box;';\ndocument.body.appendChild(el);\nconsole.log('offsetWidth  (border box):', el.offsetWidth);\nconsole.log('clientWidth  (padding box):', el.clientWidth);\nconsole.log('rect.width   (border box):', el.getBoundingClientRect().width);\nel.style.boxSizing = 'border-box';\nconsole.log('--- after border-box ---');\nconsole.log('offsetWidth:', el.offsetWidth);\nconsole.log('clientWidth:', el.clientWidth);\nconsole.log('computed width:', getComputedStyle(el).width);\nel.remove();",[768,775],{"id":769,"title":770,"slug":771,"prompt":772,"starter_code":773,"expected_output":774,"language":16,"order_index":23},128,"Apply the Global Reset","apply-global-border-box-reset","The two boxes below are both width: 250px with 20px padding and a 5px border, but they render at different total widths. Add one CSS rule that uses the universal selector (including pseudo-elements) so that every element uses border-box and both boxes are exactly 250px wide.","\u003Cstyle>\n.box { width: 250px; padding: 20px; border: 5px solid #333; background: #fef9e7; margin-bottom: 10px; }\n.big { font-size: 20px; }\n\u003C\u002Fstyle>\n\u003Cdiv class='box'>Box one\u003C\u002Fdiv>\n\u003Cdiv class='box big'>Box two\u003C\u002Fdiv>","Both boxes are exactly 250px wide in total.",{"id":587,"title":776,"slug":777,"prompt":778,"starter_code":779,"expected_output":780,"language":765,"order_index":12},"Content Width Calculator","content-width-calculator","Write a function contentWidth(width, padding, border, boxSizing) that returns the width of the content area. Padding and border are per side. For content-box return width. For border-box return width minus padding and border on both sides, but never less than 0.","function contentWidth(width, padding, border, boxSizing) {\n  \u002F\u002F TODO: implement\n}\n\nconsole.log(contentWidth(300, 20, 5, 'content-box'));\nconsole.log(contentWidth(300, 20, 5, 'border-box'));\nconsole.log(contentWidth(40, 30, 5, 'border-box'));","300\n250\n0","2026-09-30T17:00:51+00:00",{"id":503,"slug":505,"title":504},{"id":511,"slug":513,"title":512}]