diff --git a/index.bs b/index.bs
index 6abb3d2d..de33a209 100644
--- a/index.bs
+++ b/index.bs
@@ -294,6 +294,7 @@ Parts of the web platform evolve independently.
 Issues that are present with a certain web technology now may be fixed in a subsequent iteration.
 Duplicating these issues makes fixing them more difficult.
 By adhering to this principle we can make sure overall platform quality improves over time.
+
 <h2 id="api-across-languages">API Design Across Languages</h2>
 
 <h3 id="simplicity">Prefer simple solutions</h3>
@@ -490,6 +491,111 @@ See also:
 * [[#do-not-expose-use-of-assistive-tech]]
 * [[#secure-context]]
 
+<h3 id=forwards-compatibility>New web content should load in older user agents</h3>
+<!-- was "Degrade Gracefully" in the HTML Design Principles -->
+
+Design
+Web features
+so that
+they can be used
+on web pages
+and not cause
+the web page to stop working
+in user agents
+which do not (yet) implement the feature.
+
+This is called <dfn>forwards compatibility</dfn>.
+
+It is not necessary
+to consider every Web user agent ever made.
+Strong consideration should be given
+to the following categories of user agents:
+
+* Current versions of the top mainstream Web browsers.
+* Popular older versions of mainstream Web browsers.
+* Top user agents
+    designed to meet specific needs or address specialized markets,
+    such as assistive technologies or user agents targeting less typical media
+    such as text-only terminals or print.
+
+In some cases,
+a new feature may simply not apply
+to a certain class of user agents,
+or may be impractical to design
+in a way that can degrade.
+For example,
+new scripting APIs
+cannot be made to work
+in scriptless user agents.
+But in many cases,
+approaches like the following
+can be used:
+
+* A new element or attribute
+    may provide additional semantics
+    without losing essential functionality
+    when not understood.
+    This is called <dfn>graceful degradation</dfn>.
+* A new scripting method or attribute
+    can be tested before use
+    in script
+    using ECMAScript introspection facilities.
+    See [[#feature-detect]].
+* A new element or attribute
+    may provide semantics
+    and a simple default rendering
+    that can be achieved using CSS,
+    so the addition of a small stylesheet
+    allows graceful degradation.
+    <p class="example">The default presentation
+    of the <code>hidden</code> attribute
+    can be emulated
+    in older browsers
+    through the CSS rule <code>[hidden] { display: none; }</code>.
+* A new element, attribute or scripting API
+    may have behavior
+    that can be emulated
+    through the use of additional script,
+    although the scripted approach
+    may not provide
+    the same level of performance and convenience.
+    See [[#polyfills]].
+    <p class="example">The <code>getElementsByClassName()</code> method
+    can be made
+    considerably faster than
+    pure ECMAScript implementations
+    found in existing libraries,
+    but a script-based implementation
+    can be used
+    when the native version
+    is not available.
+* A new element
+    may call for
+    a highly specialized rendering,
+    but allow different content to be provided as fallback
+    for user agents that do not understand the element.
+    <p class="example">Multimedia elements
+    like <code>&lt;canvas&gt;</code> or <code>&lt;video&gt;</code>
+    allow fallback content.
+    Older user agents
+    will show
+    the fallback content
+    while user agents supporting these elements
+    will show
+    the multimedia content.
+    <p class="example">The <code>&lt;datalist&gt;</code> element
+    can be associated
+    with an <code>&lt;input&gt;</code> element
+    and may contain
+    a hidden <code>&lt;select&gt;</code> element.
+    This way
+    the fallback
+    for the intended "combo box" control
+    can be
+    a text field
+    or a text field with an associated pop-up menu
+    in older browsers.
+
 <h3 id="secure-context">Consider limiting new features to secure contexts</h3>
 
 Always limit your feature to secure contexts