Correct heading levels in Key Scenarios

key-scenarios.md

@@ -15,23 +15,23 @@ Key scenarios of the [MapML Proposal](README.md).
 - [Graceful Degradation and Progressive Enhancement](key-scenarios.md#graceful-degradation-and-progressive-enhancement)
 - [Polyfill](key-scenarios.md#polyfill)
 
-<h3 id="tiled-coordinate-reference-systems">Tiled Coordinate Reference Systems</h3>
+<h2 id="tiled-coordinate-reference-systems">Tiled Coordinate Reference Systems</h2>
 
 Perhaps the most important characteristic of Web maps that is essential for standardization, is the definition of the coordinate reference systems and the scale of the display. 
 
 ![Tiled coordinate reference systems](images/tiled-coordinate-reference-systems.png "Tiled coordinate reference systems")
 
-<h3 id="linking">Linking</h3>
+<h2 id="linking">Linking</h2>
 
-<h4 id="including-map-layers-in-html-by-reference">Including map layers in HTML by reference</h4>
+<h3 id="including-map-layers-in-html-by-reference">Including map layers in HTML by reference</h3>
 
 [Cartography](https://en.wikipedia.org/wiki/Cartography) is a challenging discipline, no matter the media in which it is performed.  The Web medium demands an entirely new kind of cartography, one which makes maps potentially more valuable than ever, by rendering them dynamically pannable and scalable, among other characteristics.  The new cartographers have a large and powerful set of server technologies, commonly known as [Geographic Information Systems](https://en.wikipedia.org/wiki/Geographic_information_system). Coupled with open data, and public server APIs, Web cartographers are able to easily publish their products.
 
-<h4 id="links-from-html-to-mapml">Links from HTML to MapML</h4>
+<h3 id="links-from-html-to-mapml">Links from HTML to MapML</h3>
 
 It’s one thing to have an element like `<layer>` to provide the client logic for a MapML document embedded in an HTML document, but what should happen if a simple `<a href="">` pointed to a MapML document?  We’ve said that a MapML document should be parseable with the HTML parser, so what should the default behaviour of activating such link be?  Probably, it should be similar to that behaviour which happens when you create a link to a media type that is natively supported by HTML, such as image/png or video/mp4: the browser synthesizes a simple HTML document with a client element such as `<img src="">` or `<video><source src=""></video>`, i.e. it should synthesize an HTML document with a `<map><layer src=""></layer></map>` element with default or generated metadata and parameters.
 
-<h4 id="links-for-layer-alternate-styles">Links for layer alternate styles</h4>
+<h3 id="links-for-layer-alternate-styles">Links for layer alternate styles</h3>
 
 ```html
 <mapml lang="en">
@@ -71,7 +71,7 @@ which replaces that layer’s browsing context with a different representation o
 
 In the above code example, a MapML document provides links to alternate ‘styles’ of itself.  The current map document is additionally and importantly tagged with the `self` link relation. This link-based facility is very powerful, because what constitutes a ‘style’ for the current map document is in the eye of the document author, and is not limited to a representation with an alternate stylesheet applied to the same data. What is linked to as a ‘style’ may be a radically different rendering of ‘this layer’, even different data, for example a satellite view vs. a topographic/street map rendering (which are obviously not simply different stylesheet renderings of the same data, as in the above example, but are in fact different data for the same extent). 
 
-<h4 id="links-for-alternate-layer-coordinate-systems-projections">Links for alternate layer coordinate systems (projections)</h4>
+<h3 id="links-for-alternate-layer-coordinate-systems-projections">Links for alternate layer coordinate systems (projections)</h3>
 
 Geographic Information Systems allow us to provide the same data in a variety of coordinate systems.  When the same layer data is available in different MapML coordinate systems, service providers can make it easy to select the right URL for a layer by encoding links to the current data represented in alternate coordinate systems, in the document metadata (in the head). The user agent can, based on the required projection designated by the `<map projection="">` attribute, automatically select (negotiate) from among the advertised links, as shown below.  The coordinate system selection/negotiation process is transparent and requires no input from the user; if the URL used by the author in the `<layer src="">` value leads to a MapML document with a coordinate system that does not match its parent `<map projection="">` value, the user agent will select the correct advertised alternate to replace it.
 
@@ -91,7 +91,7 @@ Geographic Information Systems allow us to provide the same data in a variety of
 
 `alternate` [link relations](https://html.spec.whatwg.org/multipage/links.html#linkTypes) are used to provide alternate representations by format and other criteria.  In this case, the `projection` attribute is used by the UA to distinguish the links within the link group identified by the `alternate` link relation.
 
-<h4 id="links-between-map-services">Links between map services</h4>
+<h3 id="links-between-map-services">Links between map services</h3>
 
 This feature could enable federation of map services in a way similar to the ‘federation’ of the HTML Web today, in that in today’s Web, authors decide to link to others’ Web sites and so the federation of sites constituting the Web is enabled.  The MapML-enabled Web should follow a similar path, in that not every Web map needs or can contain all the map information in the world.  So, links should be used to allow map authors to link to others maps in a similar way.  I believe this blends with or is the same as the [links between locations](#links-between-locations) use case, below.
 
@@ -109,17 +109,17 @@ Links like this could have a default visual and accessible cue to indicate link-
 
 Like `<span>` elements, `<a>` elements could appear within the `<coordinates>` element, allowing links from geometry parts.
 
-<h4 id="links-between-locations">Links between locations</h4>
+<h3 id="links-between-locations">Links between locations</h3>
 
 Links between locations could be marked up similarly to [links between services](#links-between-map-services), possibly with the addition of attributes indicating the location and zoom of the link destination. The current location of a map could be modified by activating links from one location to another.  There might need to be a different visual / accessible cue for such links.  By default, the map might animate in a “fly to” manner in response to link activation.
 
-<h3 id="graceful-degradation-and-progressive-enhancement">Graceful Degradation and Progressive Enhancement</h3>
+<h2 id="graceful-degradation-and-progressive-enhancement">Graceful Degradation and Progressive Enhancement</h2>
 
 There are many older browsers still in use on the Web, and they will likely be in use for many years to come, for a variety of reasons.  Fortunately, “client-side image maps” were very popular on the Web at one stage, and this functionality is well supported by older browsers.  For HTML authors who wish to provide a Web mapping experience for users of these older browsers, it should be possible to provide “fallback” markup that enables the core map experience they wish users to have, while providing a progressively enhanced experience for users of evergreen browsers, without relying on archaic scripting APIs.
 
 If `<area>` elements are present (for fallback) as child elements of `<map>`, they are (progressively, if the conditions warrant) treated as `<layer>` elements containing a single geographic feature, with coordinates in the `coords` attribute being interpreted as being valid pixel coordinates in the map’s locally defined map coordinate system.   More detail and a working example of [how graceful degradation and progressive enhancement could work](https://maps4html.org/Web-Map-Custom-Element/blog/progressive-web-maps.html) in this proposal is available.
 
-<h3 id="polyfill">Polyfill</h3>
+<h2 id="polyfill">Polyfill</h2>
 
 A polyfill for the [High-Level API](high-level-api.md) is available.