[
    {
        "role": "user",
        "content": "Can%20you%20give%20me%20the%20w3c%20good%20example%20for%20filters"
    },
    {
        "role": "assistant",
        "content": "Could you clarify which kind of \"filters\" you're asking about? W3C has several contexts for filters, such as:\n\n- CSS / SVG filter effects (e.g. blur, color adjustments)  \n- Accessibility filters or guidelines (WCAG)  \n- Content filtering systems  \n- Other kinds of filters (like in APIs or data queries)\n\nMeanwhile, here\u2019s a good example of W3C\u2019s CSS / SVG filter effects from the W3C Filter Effects Module, plus an explanation:\n\n---\n\n## Example from W3C: CSS / SVG Filter Effects\n\nFrom the *Filter Effects Module Level 1* (W3C Recommendation), filters are operations applied to the rendered output of an element (including children) **before** it\u2019s composited into its parent. These are used for effects like blurring, adjusting color, warping, etc. ([w3.org](https://www.w3.org/TR/filter-effects-1/?utm_source=openai))\n\nExample usage includes:\n\n```css\n.element {\n  filter: blur(5px) contrast(200%);\n}\n```\n\n- This applies a **blur** of 5 pixels, then increases **contrast** to 200%.  \n- Multiple filter functions are applied in the order they appear. ([developer.mozilla.org](https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Properties/filter?utm_source=openai))\n\nOther valid filters include: `grayscale()`, `hue-rotate()`, `sepia()`, `invert()`, `opacity()`, `drop-shadow()`, etc. ([developer.mozilla.org](https://developer.mozilla.org/en-US/docs/Web/CSS/Reference/Properties/filter?utm_source=openai))\n\nIn SVG context, the spec provides `<filter>` elements with filter primitives (like `<feGaussianBlur>`, `<feColorMatrix>`, `<feOffset>`) to build more complex filter effects. ([en.wikipedia.org](https://en.wikipedia.org/wiki/SVG_filter_effects?utm_source=openai))\n\n---\n\nIf you tell me which kind of \u201cfilters\u201d you mean (CSS / accessibility / image / data / etc.), I can pull up the **good example** you\u2019re looking for."
    },
    {
        "role": "user",
        "content": "%20Other%20kinds%20of%20filters%20(like%20in%20APIs%20or%20data%20queries)"
    },
    {
        "role": "assistant",
        "content": "Here are some good examples and best practices from W3C and IETF-type specs of how to use filters in APIs or data queries. These examples show how filtering query parameters are structured, encoded, and combined to allow flexible, expressive queries.\n\n---\n\n## 1. RESTCONF (RFC 8040)\n\n**RESTCONF** is a standardized protocol for network configuration and state data using HTTP, built atop YANG data models. It defines several query parameters including `filter` to restrict the subset of events or resources returned. ([rfc-editor.org](https://www.rfc-editor.org/info/rfc8040/?utm_source=openai))\n\n### Key filter parameter rules:\n\n- `filter` is used only with GET on **event stream** resources. ([rfc-editor.org](https://www.rfc-editor.org/info/rfc8040/?utm_source=openai))  \n- The value of `filter` is an **XPath 1.0 expression**, evaluated with specific context: namespaces, nodes, etc. ([rfc-editor.org](https://www.rfc-editor.org/info/rfc8040/?utm_source=openai))  \n\n### Example URIs:\n\nTo get only events of class \u201cfault\u201d:\n\n```\nGET /streams/NETCONF?filter=%2Fevent%2Fevent-class%3D'fault'\n```\n\nTo get events with severity \u2264 4:\n\n```\nGET /streams/NETCONF?filter=%2Fevent%2Fseverity%3C%3D4\n```\n\nTo filter by module + status:\n\n```\nGET /streams/NETCONF?filter=(%2Fexample-mod%3Aevent1%2Fname%3D'joe'%20and%20%2Fexample-mod%3Aevent1%2Fstatus%3D'online')\n```\n\n*(Note: strings in the URI are percent-encoded.)* ([rfc-editor.org](https://www.rfc-editor.org/pdfrfc/rfc8040.txt.pdf?utm_source=openai))\n\n---\n\n## 2. RFC 008: API Filtering (Wellcome Collection)\n\nThis is a general-purpose recommendation for consistent filtering/sorting in APIs. It\u2019s not W3C but useful for REST-style APIs dealing with resources. ([docs.wellcomecollection.org](https://docs.wellcomecollection.org/request-for-comments-rfcs/008-api_filtering?utm_source=openai))\n\n### Key patterns:\n\n- **Structured filtering**: Using query parameters named after resource properties.  \n- Combining multiple parameters \u2192 logical **AND**.  \n- Multiple comma-separated values within one parameter \u2192 logical **OR**.  \n- Support for ranges, dotted path syntax. ([docs.wellcomecollection.org](https://docs.wellcomecollection.org/request-for-comments-rfcs/008-api_filtering?utm_source=openai))\n\n### Example style (pseudo):\n\n```\nGET /works?type=book,ebook        # type == book OR ebook\nGET /works?subject=history        # subject == history\nGET /works?type=book,ebook&subject=history\n```\n\nFull text search via `query` parameter: e.g.,\n\n```\nGET /works?query=skeletons&type=book,ebook\n```\n\n---\n\n## 3. W3C\u2019s own API for W3C Data (pagination, \u201cembed\u201d, etc.)\n\nThe official W3C API (api.w3.org) uses simpler filters/control parameters rather than full boolean filters:\n\n- `items` \u2014 how many results per page (max 1000) ([w3.org](https://www.w3.org/api/?utm_source=openai))  \n- `page` \u2014 which page of results ([w3.org](https://www.w3.org/api/?utm_source=openai))  \n- `embed` \u2014 boolean, when wanting related resources embedded rather than as links ([w3.org](https://www.w3.org/api/?utm_source=openai))  \n\nThis is common when heavy filtering isn\u2019t required, using simple query parameters rather than complex expressions.\n\n---\n\n## 4. OGC API \u2013 Features: Part 3: Filtering (for geospatial / \u201cfeatures\u201d APIs)\n\nThe **OGC Filter standard** (for feature APIs) defines a `filter` query parameter. It allows clients to use predicate languages to define which resources should be returned. ([docs.ogc.org](https://docs.ogc.org/is/19-079r2/19-079r2.html?utm_source=openai))\n\n---\n\n## Best Practices / Patterns\n\nFrom the examples above and general API design guidance:\n\n1. **Use query parameters** for filters: more RESTful than encoding filters into the URI path.  \n2. **Make operators explicit**: equality (`=`), inequality (`!=`), less/greater (`<`, `<=`), substrings/contains.  \n3. **Support logical combinations**: AND, OR, grouping with parentheses where needed.  \n4. **Percent-encode special characters** in URIs, especially if using reserved characters.  \n5. **Limit syntax complexity where possible** for clarity. If you need heavy querying ability, consider adopting standardized filter/query languages (XPath, OData, etc.).  \n6. **Document clearly** which fields are filterable, operators supported, and any limitations (e.g. only certain endpoints support `filter`).  \n\n---\n\nIf you like, I can pull up a *template* or *spec-compliant example* tailored to your kind of API (e.g. REST with JSON), showing how to design filters. Do you want that?"
    }
]